Are my DC old-enough, big-enough for no car seat/booster?

[scarecrow22]

Forgive me, but I am so confused and can't even find anything on Google which makes me confident about this.

DD is just 10yo, is nigh-on 5ft and weighs 41kg. Does she still need a booster seat?

DS is nearly eight, and is average height and build (it would be better if I'd measured him, but every time I try to do this in the day I'm sat on, or there's a meal to cook, or DH talks to me, or the battery runs out..) Does he need a booster with a back, or just seat, or no booster?

All advice welcome, but if you are quoting from any authority I'd love to know who or where.

I thank you.

In the UK its 135cm or age 12 whichever comes first, for needing any kind of booster seat

My 7 year old is 142cm, he has just stopped using his seat because it was getting uncomfortable for him. He is absurdly tall for a 7 year old though.

145cm minimum to be safe in a standard seat belt.

5 ft is 150cm so it's unlikely your 10yo needs a booster.

8yo average height is 128cm, so it's likely he still needs a booster. Legally they need one until 135cm or 12 years, although usually they will fit a seatbelt better with one until some point between then and 150cm. Adult seatbelts are designed for 150cm+ although some children in some cars will fit without a booster earlier.

To check if they fit a seatbelt correctly without a booster you sit them in the car and see where the seatbelt passes over. It needs to pass over their collarbone at the diagonal belt, and the lap belt should run across the hips or very tops of thighs. And they need to be able to bend their knees over the edge of the seat with the seatbelt in a good position.

What you want to avoid is the belt going over their tummy as this is very soft and will likely cause internal bleeding if you were to crash. And if the seatbelt touches their neck then they will get annoyed with it.

8yo is tall enough for a backless booster, but one with a back on will probably position the belt better on his neck, will support him better if he falls asleep (unlikely unless you do long overnight drives TBH) and it will offer more side impact protection in a crash.

Oh sorry forgot to add sources:

Info from RoSPA about UK law about car seats/booster seat use in general:
childcarseats.org.uk/the-law/

More info from RoSPA about booster cushions:
childcarseats.org.uk/types-of-seat/booster-cushions/

Info from Britax about the difference in safety between a booster cushion and full/highback booster:

This is a bit overly detailed and also American but also applies: Guidelines for checking if a child fits the adult belt without any booster seat:
csftl.org/the-five-step-test/

Too many different articles/studies to link about "seatbelt syndrome" aka the injury pattern seen in children where the belt sits too high up on the abdomen. But here's one for example:
dontforgetthebubbles.com/seat-belt-injuries/

Thank you all SO much. Brilliantly clear. At last. Honestly, looking at any one source is so confusing as there is always lack of clarity somewhere, or a gap, and then I look at another source and there appears to be a conflict in advice! @BertieBotts you are a legend for synthesising all that information. Thank you all.