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Best thermal underwear and base layers to keep kids toasty this winter

Keep your kids toasty with our roundup of the best thermal tops, leggings and base layers to add to their winter wardrobe.

By Lucy Cotterill | Last updated Oct 17, 2022

Polarn O Pyret kids thermals

A thermal layer worn under everyday clothes gives children the freedom to play outside in winter without getting too cold. Thermal underwear is also indispensable for camping, snow days and skiing holidays, keeping kids cheerful and comfy in even the coldest conditions. 

During a cost of living crisis and facing a significant increase in energy bills, many of us are also considering thermals for our family for more everyday use, adding an extra layer to help keep us warm in the home without turning up the thermostat.

A far cry from the itchy thermal vests of old, today’s thermal base layers are smooth, soft and comfortable and can easily be worn as extra-cosy versions of everyday tops and leggings, or as pyjamas for the coldest nights of the year. 

A good base layer is worth its weight in gold, but they’re not all created equal. We’ve thoroughly investigated the best kids' thermals as recommended by real parents in the Mumsnet forums. 

Whether you’re looking for merino wool base layers, a thermal top or budget-friendly thermals for your children, here are the best kids’ thermal underwear and base layers you can buy today. 

1. Best overall thermal underwear for kids: Uniqlo Kids HeatTech Ultra Warm Crew Neck Long Sleeved Thermal Top

Uniqlo Kids HeatTech Ultra Warm Crew Neck Long Sleeved Thermal Top

Price: £14.90 | Buy now from Uniqlo 

Uniqlo Heat Tech leggings and tops worn under winter clothes.”  (Tried and tested by Mumsnet user LIZS)

Uniqlo heat tech. Buy them every year and we’ve used them twice in Finland in winter when it’s been very cold.” (Recommended by Mumsnet user Onthetrain75

When it comes to thermals, one brand is mentioned time and time again by Mumsnet users - Uniqlo HeatTech - so much so that it’s the top choice for both our children’s and women’s thermal selection. 

Made from smooth and stretchy fabric that is soft to the touch, the Uniqlo HeatTech thermal tops are insulating and moisture-wicking, helping keep your child warm and comfortable even when they don’t keep still.  

With a crew neck design and long sleeves, this Ultra Warm edition is ideal for wearing as an inner layer or t-shirt on extra cold days and is over two times warmer than the regular Heattech thermals. 

Even better? Compared to some other kids' thermal brands, it offers an affordable price point too, meaning you can pick up a spare to have whilst the other is in the wash. 

Pros

  • Soft and stretchy
  • Extra insulating for cold winter days 
  • A discreet crew neck design can be worn under other clothing 

Cons

  • Only comes in two colour choices 

Key specs

  • Material: 44% Polyester, 32% Acrylic, 21% Viscose, 3% Elastane
  • Colour: Black or white 
  • Size: From age 3-4 to 13 years 

2. Best budget thermal underwear for kids: Decathlon Kids Base Layer Top

Decathlon Kids Base Layer Top

Price: £3.99 | Buy now from Decathlon

“We get all our ski base layers from Decathlon. Usually can get a set of bottoms and top for less than a tenner”  (Tried and tested by Mumsnet user Pertella)

“Decathlon have base layers for a few quid.” (Recommended by Mumsnet user TheYearOfSmallThings

If you’re looking to pick up some extra layers for your children this Winter but some of the eye-watering price tags are putting you off - fear not. Decathlon has a fantastic range of budget-friendly base layers that you can pick up for just a few pounds. You can even pick up the matching leggings to get a full set for less than a tenner. 

Available in a choice of black or white, this kids' base layer top is made from brushed polyester for extra warmth and comes with lie flat seams - ideal for layering under other clothing without adding bulk. 

Whilst the turtle neck makes it a little less discreet as a base layer under regular t-shirts and jumpers, it offers extra protection around the neck, an area which can easily catch a chill during those icy school runs or whilst playing sports outside. 

Pros

  • Budget-friendly price tag
  • Matching leggings available
  • Lie flat seams 

Cons

  • Turtle neck may not be the most versatile choice 

Key specs

  • Material: 100% Polyester
  • Colour: Black or white
  • Size: 4 years to 14 years 

3. Best thermal base layer for kids: Lands End Thermaskin Thermal Long Johns

Lands End Thermaskin Thermal Long Johns

Price: £12 | Buy now from Lands End

“Lands End have some good stuff. They are American so have stuff suitable for extreme cold.”  (Tried and tested by Mumsnet user Yewtown)

“Lands End does some very nice fleece lined leggings and joggers. Good quality and wash well..” (Recommended by Mumsnet user Jellycatspyjamas

If your child begs to play outside on snowy days, only to come in five minutes later saying they’re cold - they clearly haven’t tried Lands End Thermaskin Long Johns.

These easy-on thermal trousers with an elasticated waist are made from a super soft and stretchy jersey fabric, which generates warmth to keep them toasty all afternoon. 

Lightweight with flat locked seams, they can easily be layered under joggers or waterproof trousers, whilst their moisture-wicking properties will ensure they don’t sweat or start to overheat.

Unlike some other thermals, the Lands End designs are also really appealing to kids - with bright and colourful designs instead of the (somewhat boring) white and black. 

Pros

  • Stretchy and comfortable jersey
  • Moisture wicking for active days
  • Bright, colourful designs 

Cons

  • Not available in preschooler sizes 

Key specs

  • Material: 90% polyester/10% elastane
  • Colour: Deep Blue Doodle Stripe (blue) or Verbena (pink) 
  • Size: 4-5 years to 14 years 

4. Best merino wool thermal underwear for kids: Polarn O. Pyret Thermal Merino Wool Kids Top

Polarn O. Pyret Thermal Merino Wool Kids Top

Price: £26 | Buy now from Polarn O. Pyret

“My boy has some wonderful merino ones from Polarn O Pyret - while they cost an arm and a leg, they do a good job of keeping both warm.”  (Tried and tested by Mumsnet user ojell)

Whilst it tends to come with a slightly higher price tag, merino wool is the best fabric you can buy for your kids’ thermals, not only because it’s naturally soft, but also offers the ideal balance of warmth and moisture management. 

These merino wool thermal tops from Swedish brand Polarn O’ Pyret are available in a choice of six fun shades, from the subtle light grey to bold and bright pumpkin orange. They’re also suitable for babies as young as six months through to 12 years - ideal if you like to embrace a bit of twinning between siblings.

Made from 100% merino wool, these base layer tops are perfect for layering on colder days, leading cold moisture away from the skin whilst retaining body heat while still feeling soft and non-itchy against young skin. 

Pros

  • High-performance merino wool
  • Range of fun, colourful designs
  • Moisture wicking 
  • Available in smaller sizes

Cons

  • Costly 

Key specs

  • Material: 100% Merino Wool
  • Colour: Choice of six colours 
  • Size: From 6-12 months to 10-12 years 

5. Best thermal leggings for kids: Mountain Warehouse Merino Kids Thermal Baselayer Trousers

Mountain Warehouse Merino Kids Thermal Baselayer Trousers

Price: £14.99 | Buy now from Amazon

Mountain warehouse thermals are great”  (Brand recommended by Mumsnet user PeasAndHarmony)

“Mountain Warehouse is good too core base layers. We're in bloody freezing Scotland and the base layers, ski jackets and socks etc are life savers for school and outdoor play.” (Recommended by Mumsnet user WhereverIGoddamnLike)

If you’re heading out on a winter walk with your family, these Merino Kids Thermal Baselayer trousers can easily prevent your children’s legs from getting cold.

Made from merino wool and a polyester blend, these lightweight and comfortable thermal trousers can not only help retain heat but are breathable too, keeping your child dry and comfortable even on more active days. 

With a stretchy elasticated waist and cuffed ankles, they add a waterproof protective base layer for your children without adding bulk and come in a range of fun colours and shades. 

Pros

  • Extra warm yet moisture wicking
  • Soft and comfortable waistband 
  • Waterproof and breathable 

Cons

  • Whilst we love the bright shades, some tweens or teens may prefer a more subtle colour. 

Key specs

  • Material: 100% Cotton
  • Colour: Choice of six colours 
  • Size: From 2-3 years to 13 years 

6. Best lightweight kids’ thermals: M&S Thermal Vests

M&S Thermal Vests

Price: From £12 for two | Buy now from Marks and Spencer

“M&S do some nice thermals on multi-packs. Long sleeved tops and under trousers.”  (Tried and tested by Mumsnet user Jonsey79)

Offering great value for money, this pack of two M&S Thermal Vests can quickly become a winter wardrobe essential for your kids.

Made from super soft brushed fabric with lie flat seams, these long-sleeved scoop-necked tees are perfect for layering under a school uniform or regular clothing to create a cosy base layer on colder days.

With a pack of two for just £12, you can always have one in circulation whilst the other is in the wash. 

Pros

  • Soft and cosy brushed fabric 
  • Non-bulky lie flat seams
  • Available in larger sizes  

Cons

  • Not as warm as merino wool thermals  

Key specs

  • Material: 50% cotton, 25% viscose and 25% polyester
  • Colour: White, black, blue mix and lilac mix (sold separately) 
  • Size: From 2-3 years to 15-16 years  

7. Best kids’ thermal top: Sports Direct Thermal Baselayer Top Unisex Junior

Sports Direct Thermal Baselayer Top Unisex Junior

Price: £6.50 | Buy now from Sports Direct

“The Sports Direct own brand ones are very warm, and cheap.”  (Tried and tested by Mumsnet user Flomper)

“My son wears a base layer under his strip when playing football. I got it in sports direct he says it keeps him cosy.” (Recommended by Mumsnet user alwaysstressed

If your kids enjoy playing sports at the weekends, but you’re worried about them getting chilly whilst out on the field, these Thermal Baselayer Tops from Sports Direct are ideal for layering underneath their kit.

Available in a choice of colours, this unisex design baselayer is a soft and breathable crew neck with long sleeves and ribbed cuffs for comfort. Keeping them warm whilst drawing moisture away from their body,  they can burn off excess energy without overheating. You on the other hand may be shivering on the sidelines (so you may want to pick up some thermals of your own too!) 

Pros

  • Affordable price
  • Moisture wicking
  • Range of colours 

Cons

  • Material non specified 
  • Not available in smaller sizes 

Key specs

  • Material: Unspecified
  • Colour: Choice of five colours 
  • Size: From 7-8 years to 13 years

8. Best kids’ thermals for skiing: Icebreaker Kids' Merino 260 Thermal Leggings

Icebreaker Kid’s Merino 260 Thermal Leggings

Price: £45 | Buy now from Icebreaker

“Look at Icebreaker thermals they come in different weights they even have summer weight tops for sport. When skiing I would usually go for 180 weights but when I went a particularly cold year (-15plus) had one 220 or 240 can’t remember but too hot for ‘normal’ winter skiing. Kids have had a combination of mountain warehouse and Icebreaker”  (Tried and tested by Mumsnet user snowballupahill)

If you’re heading skiing or to arctic weather conditions this winter, then you may want something slightly thicker than your average thermals. These Merino 260 Thermal Leggings come with a thicker weight rating, ideal for extra cold climates.

With a brushed elasticated waistband for comfort and flat lock seams that won’t rub or chafe, these leggings are made from 100% merino wool, renowned for their warmth and temperature regulation. 

Ideal for skiing, hiking or layering up in cold weather, they can keep your child warm, dry and comfortable in even the most bitter of weather conditions

Pros

  • 100% merino wool
  • Thicker for colder climates 
  • Moisture wicking to prevent sweat or odour 

Cons

  • More expensive than some
  • Sizing is based on European sizing - so you’ll need to use the measurement size chart to ensure you select the right size. 

Key specs

  • Material: 100% Merino Wool
  • Colour: Choice of three colours 
  • Size:  European sizes 2 - 14 (roughly 2-3 - 11-12 years) 

9. Best kids’ thermal set: Trespass Kids' Unite360 Warm Thermal Base Layer Set

Trespass Kid's Unite360 Warm Thermal Base Layer Set

Price: From £14.99 | Buy now from Amazon

“I just got some good stuff from Trespass.”  (Product range recommended by Mumsnet user HesterShaw1)

Whilst many kids' thermals are sold individually, you can also pick up full kids' thermal sets, combining a matching top and a pair of long johns or leggings. 

This Warm Thermal Base Layer Set from trusted outdoors brand Trespass come at a really affordable price and includes a longer-length zip neck base layer top with long sleeves and full-length leggings, with an elasticated waist and deep cuffs to the ankle.

Made from moisture-wicking and fast-drying polyester, the set provides good coverage to keep your kids warm and dry when the temperatures drop. 

Pros

  • 2-in-1 set
  • Affordable price tag
  • Moisture-wicking and breathable 

Cons

  • Not all kids will like the zip at the neckline 

Key specs

  • Material: 100% Polyester
  • Colour: Black
  • Size: Age 3-4 to 11-12 years 

10. Best thermal socks for kids: Heatholders Thermal Heat Socks

Heatholders Thermal Heat Socks

Price: £5.99 | Buy now from Amazon

“I've got 'heat holders'. I bought them on amazon, but have also seen them in Primark. I think they do child-sized ones too. They are super warm.” (Tried and tested by Mumsnet user TheFormerPorpentinaScamander)

When it comes to keeping your kids warm and comfortable, there is nothing worse than cold feet. These thick Heatholders Thermal Socks are the ideal solution to keep your kids' toes toasty this winter. 

They’re brushed inside, so super soft against your child’s skin, whilst they offer a warmth rating of 2.34 togs (compared to around 0.4 togs for a traditional cotton sock). 

Longer length and ideal to wear with wellies, it’s always good to have a pair or two handy for snow days or the sudden onset of cold weather. 

Pros

  • Longer length socks
  • Super thermal with a high tog rating

Cons

  • Not the most discreet of colours 

Key specs

  • Material: 91% Acrylic, 5% Polyamide, 3% Polyester and, 1% Elastane
  • Colour: Pink
  • Size: 4-7 years and 9-12 years

What to look for when shopping for kids’ thermals 

There are lots of kids’ thermals on the market, so here are some things to consider when making your choice: 

  • Weather conditions - Where is your child going to be wearing their thermals? Are you heading off on a ski holiday or simply looking for everyday thermals to keep them warm under their school uniform? If you’re visiting extremely cold climates, you may need to opt for thicker, warmer fabrics such as Merino wool, whereas polyester mixes will generally do the job for day-to-day use.

  • Activity level - Will your child be exercising or playing outdoors in their thermals? If you’re looking for thermals to use during sports, walks, hikes or skiing, then it’s recommended to invest in moisture-wicking and breathable fabrics that will prevent your child from overheating. 

  • Cost - You don’t have to spend a fortune for high-quality thermals that are warm enough for a winter base layer. That said, if you are looking for high-performance fabrics that offer significant warmth, heat generation, odour protection, antibacterial linings and breathable, moisture-wicking properties, you may need to spend a little more. 

  • Layering - Will your child be wearing their thermals as a base layer underneath other clothes? Some cuts work better for use as a base layer with lie-flat seams and scoop necks that are discreet when used for layering. Others have zips and high collars, and as such, are better suited for more extreme weather or wearing as a top in their own right.   

Does thermal underwear really work?

In a word, yes! Thermal underwear is made from fabrics designed to trap heat and draw moisture away from the skin, so having a base layer under normal clothes really does make a difference. 

When should kids wear thermal base layers?

If your children are going to be outdoors for a long period of time in cold weather, such as walking or playing sports, a base layer is a sensible idea to ensure they don’t feel exposed to the elements. 

They’re also ideal as an every day extra layer if you’re attempting to reduce your heating bills. For winter activities such as hiking and skiing, they’re an absolute must, keeping your child warm, dry and comfortable, no matter how low the temperatures drop.  

What’s the best material for kids’ thermals?

Merino wool is the gold standard for thermals, renowned for its warmth and moisture wicking properties. That said, it can be expensive, so polyester blends can also keep your kids toasty with a far smaller price tag

What’s the best kids’ thermal underwear to buy?

Uniqlo Kids HeatTech are the best kids’ thermals to buy today. We love them for their simple designs, discreet necklines that are convenient for layering, and their soft touch and cosy fabrics. 

How we chose our recommendations

Recommendations from real Mumsnet users 

Lots of Mumsnet users are concerned about keeping their kids warm this winter. With rising energy bills and the increased cost of living, there have been loads of discussions about ways to keep costs down, from hot water bottles and hoodie blankets to thermals and fleece pyjamas

First, we sought out the trusted opinions of Mumsnet users, to get their feedback on the kids thermals they rate, recommend and have used with their children. 

We also looked out for any negative reviews, where Mumsetters highlighted products that they felt were poor quality or over priced.

Authentic consumer reviews

Finally, to bring you our definitive list of the best kids' thermals on the market, we thoroughly investigated genuine, in-depth customer reviews on retailer sites such as Amazon, Marks and Spencer and John Lewis to get wider consumer experiences regarding the products on our shortlist. 

Why you should trust us

We work hard to provide honest and independent advice you can trust and we spend hours scouring the Mumsnet forums and online bestseller lists to find products that real parents rate.

Transparency is really important to us and that's why we're always upfront about where we find our recommendations. We write about products that we feel offer the best value to most parents – the ones that our users would recommend to their own friends and family.

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