When to stop using car seats?

[cautiouscovidity]

DC1 is 10 and very small for their age and DC2 is 9. At 134cm (both kids are the same height!), they've almost reached the height at which they no longer HAVE to use a car seat but should we stop (both are still in a high backed booster that say they're suitable up to 250cm / 36kg which mine are nowhere near that limit)?
I'm aware that most of their friends haven't used a car seat if any kind (even a booster) for years (many of which were below the height but hey ho).
Im thinking maybe switch to just a booster for round town trips, but use the high back for long / motorway journeys (not that we've been on any of those for a while 😂)?

DS2 is 11 and plenty tall enough, but also nowhere near 36kg. Fortunately he still prefers it, so it's staying.

DS was 10, 150cm and didn't fit comfortably in it any more. DD9 is still in hers and will be until she hits 150 too. But we don't get a choice, it's the law where we live although most of their friends are on a simple booster. DD still falls asleep in the car, so it's useful for that too!

DSS is 8 but very tall. He hit the 135 cm when he was about 6. We left him in it for another year or so then let him go without one. His mum let him out of one once he was 135 cm.
The reason we left him in a bit longer is it's our car that he did the motorway journeys in between the two houses so figured why not.

My DD is very tall and reached 135cm before she turned 8. We kept using it until she was too big for it which was just after her 8th birthday.

We bought a booster with isofix for 9yo. He's over the height and weight requirements but we intend to use it until he's around 12

At 135cm they are legal without any kind of car seat but they are NOT safe. Car seats and seatbelts are designed to fit men (not women, typical!) of at least 150cm. The risk of 'seatbelt syndrome' i.e. life-changing or fatal internal injuries caused by the seatbelt itself in a crash is too high to go without a car seat until a child is 145-150cm.

My DD is 10, probably 135cm+ by now (haven't measured for a while!) and I have very recently consented to her using a backless booster for local journeys. She knows that, if we needed to go on a motorway, she'd still be in the high back booster. I'd prefer she was still in it 100% to be honest, but we rarely drive anywhere due to lockdown/restrictions and very rarely exceed 20mph on the rare and very local journeys that we still do.

If I put her on the seat without a booster, it's blatantly obvious that the belt does not fit her. It's worth having a read of this if your kid(s) are in the 135-150cm height range: saferide4kids.com/blog/take-the-5-step-seat-belt-test/

Also worth remembering that cars differ; for example in ours a 140cm kid would probably be fine in the back row without any seat. In the middle row it would be very iffy with regards to the belt fit. In the front seat, with no booster, they would be at direct risk of serious injury from the seatbelt and also the airbag, which would fire pretty much in their face/neck and not their chest.

Honestly unless your child is VERY tall and VERY well-built I would keep using at least a backless booster until they are 12.

Those parents letting 130-135cm tall kids sit (illegally, if under 135cm!) without boosters have obviously never googled 'seatbelt syndrome'. And those letting them do it in the front seat have definitely never googled 'child airbag injuries'.

Yes, it's very unlikely that you will be involved in a car crash. Especially if you drive carefully and defensively. But you just never know when an accident might happen and you never want to live to regret not having used the most appropriate child seat.

Those countries (much of Europe, including the ROI!) that insist on booster use until 150cm do so for good reason.

Our car seat laws in the UK have improved a little in recent years. But they are an absolute minimum, and your child won't be as safe as they should be if you follow them to the letter. E.g. to protect their little necks, it's best to be rearfacing until age 4-6 whereas it's legal to have them forward facing from 15 months. (Used to be 9kg whatever their age, which meant babies under 1 being put in forward facing seats!) And for boosters, some think that because 135cm is the law then it's PERFECTLY safe then and if they're nearly 135cm it'll be fine. Which couldn't be further from the truth, unfortunately.

@NotWithMyShoes

DS was 10, 150cm and didn't fit comfortably in it any more. DD9 is still in hers and will be until she hits 150 too. But we don't get a choice, it's the law where we live although most of their friends are on a simple booster. DD still falls asleep in the car, so it's useful for that too!

This is really relevant too, if they're going to fall asleep then they will simply fall out of the shoulder belt on a backless booster. More likely to 'submarine' (have the lap belt ride up too high) too. Another reason for hanging on to high back boosters as long as possible. :-)