Roll your clothes is my only tip. You get so many more in then.

Lay everything out on the bed and make sure your clothes mix n match.
Don’t take things ‘just in case’.
Be ruthless - Ive just packed for four days away and am taking one cardigan and a raincoat,
Take layers eg a vest t-shirt that can go under a shirt if necessary.
Take the bear minimum of toiletries.
I agree with rolling.
Use a rucksack as it leaves your hands free for buggy and suitcase.

Just don’t take everything

For three year old I would be aiming for 4 basic outfits like t shirt and leggings. Wash as needed.

Compression packing cubes, they are pricy but really do save room. Make sure you buy compression ones not normal ones which are purely for sectioning and organisation.

Take dark shorts as they don’t show the dirt
Avoid denim - it takes up space - take cotton/Lycra leggings for colder days, live in shorts the rest of the time. Maybe a pretty casual cotton dress or skirt.
Travel in a vest top and shirt/sweater. Vest top becomes your pj top for first half of the week.
One pair of lightweight summer trainers or similar, and one pair of sandals you can walk in and wear on the beach. If the sandals don’t go with any of your skirts/dresses, don’t take a skirt/dress, take a pair of nice shorts instead.

For beachwear, I always take a few cotton shirts I can throw over what I’m wearing, and microfiber towels for the beach. I’d rather have several thin towels like that as they dry really fast.

you can rinse out socks, bras and knickers by hand, and re-wear once dry. Other clothes I would spot clean, air and wear twice. I usually take a long piece of string and cross fingers I can find some way of rigging up a little washing so I can make sure swim kit dries well overnight.

Microfiber towels are great for space saving. Packing cubes don't need to
Be expensive. I like mountain hardware / warehouse ones on amazon.

Wear your heaviest, bulkiest clothes. If packing shoes, fill them with stuff, eg underwear, tubes of toothpaste or bottles of shower gel etc. A scarf can be an extra layer,or hide a stain to give you an extra day out of something etc.

Vacuum bags? We’ve got some from Amazon with a manual hand pump so no need for hoovers, good for taking the air out of thicker stuff / nappies etc. doesn’t help with weight but saves a lot of space. Then on the way back i
pile the dirty clothes in and shrink then down for easy packing.

bar soaps and solid toiletries - solid shampoo and solid soaps inc Johnson’s or Dove baby soap - cut a chunk off even and take that, a small amount will see you through a week easily. Much less space than bottles.

When I travelled abroad with my 1yo in a buggy, I found that a wheel able suitcase was actually easier with the changing bag on the pram and I had rucksack on me and a small hand luggage bag on top of the suitcase that strapped on. The suitcase was way easier to manoeuvre around than the wheel bag I tried initially as it was prone to falling over and not as easy to manhandle.

If I'm honest, I wouldn't be taking a pram for a 3yo, partly because I think that it would be harder work and unnecessary for a 3yo,but also because Cornwall is a nightmare with a pram, for the hills etc.

I presume that the 3yo is out of nappies, so beyond the absolute necessary foods for travel down and any dietary specific foods, I'd be googling supermarkets and go to the supermarket once you've arrived. Or order an online delivery for once you've arrived. I've known friends to even order to a hotel room, tipping up with bottles of wine to get the minimum spend!

Re packing, only take one lot of beach items. When my child was a toddler I found roll up trousers fab, as could have as shorts or trousers in the same day. I have always found that patterned/pictured clothes fair better than plain. Opt for items that are easy to keep together, such as sunhats or caps that can be clipped/attached to your bag etc. Definitely take a couple of outfits that would be for much colder days with lighter layers underneath, it's always colder in the evenings and frequently so during the days.

For toys, I used to take a water book where there were activities and colouring that were revealed using the pen you filled with water and then dried and repeat.

Have fun.

Other than food for the journey and maybe something light such as tea bags, ditch the food and book an online shop for delivery when you arrive if you’re self catering. Or shop when you get there.
ruthless packing and wash while you’re there if needed. Travel in your bulkiest clothes (jeans, fleece or jacket and trainers).

Give a little backpack to your DC for their toys and bedtime snuggle/bear.

Where are you staying? Ability to do laundry will make a huge difference to how little you can get away with. I agree with comments re ruthless minimal packing…one pair of trainers on your feet that will do every occasion etc.

I’d assume that the only things to take food and supplies wise are the things that you would use in minute quantities e.g. a tiny bottle of oil, a handful of dishwasher tablets. Everything else can be bought once there or maybe have an internet shop delivered to where you are staying?

I agree with PP re packing cubes being really helpfu, not just for their compression capability but because it makes it so easy to be organised and to unpack.

Superdrug is really good and much cheaper that boots for tiny supplies like deodorant and heat protector. Has all the standards like shampoo, conditioner, toothpaste etc too but I always have little ones of these from previous hotel trips. These plus minimal make up (ie one each of essentials and no choices) can get in an airport hand luggage sized cosmetic bag.

Locate a local laundry, and put in a service wash if required.

If you have a pushchair I'd utilise that and put a smaller hold-all underneath and then a bag on the handle and a rucksack rather than one huge suitcase.

If your staying with friends or self catering hopefully id you can do washing that'll make a huge difference x

Totally agree with ZenNudist about the quick drying towels. They take up a lot less room . However I would also like recommend something I’ve just bought for my little one. It’s a Go Goosey quick drying onesie. We bought for my daughter when we went on our recent holiday to Spain. It was an absolute godsend! Not only does it double as a towel which means you can save on valuable suitcase space, it also dries them while wearing it and kept her warm and cosy as soon as she got out of the pool/sea. She lived in hers on the beach and in the hotel room in fact we had trouble getting her out of it. 😂 it also stopped all the drying tantrums.

Bought mine online at gogoosey.co.uk
Not sure if they're in any stores yet?