Do other parents see car seats just as cosmetic / a polite courtesy?

I think they have a different capacity for assessing risk and impact to you.

With my older DC (8) any lifts offered have always been on the premise a car seat is dropped off. Or it’s a “we have one spare car seat so can someone bring another please”

No idea about average heights but both mine reached the height tall enough to be out of car seats when they were age 7. Before that I did have a travel booster seat for various holiday situations/ as a spare for friends. Maybe get one of those.

My daughter is 10 and still in a HBB. She is no where near tall enough to go in a car without a car seat. I do normally take her to any party where the host offers to drive as I don't trust them to accommodate the car seat (if they are taking 4 kids unless they have a big car, it can be a struggle to get the seat and others in comfortably). I always think I would never forgive myself if I trusted someone else on it and they ignored the seat for whatever reason.
I do feel that this is very much a me issue and other parents have always felt that I am weird (having seen their photos of multiple children who should be in car seats but clearly without, I do not doubt my instinct though).

I don't think she's saying she has a problem with doing that, just that she hadn't thought of it as an option! Presumably it's not what any of the other parents do around OP which is surprising.

We've always been really strict on car seats and would ask other driver to use our car seat at that age rather than just a seatbelt or even a booster. Especially is going on duel carriageway. The evidence of what a difference an age appropriate car seat makes is undeniable. I actually can't believe how slack the laws are in the UK.

In the first scenario it is a one-off where you wouldn't expect to have a spare car seat. Some parents will judge that a short journey at age 7 without a car seat is safe, others won't. You didn't, so it is fine. In the second scenario, I would have thought you just give them your car seat to use. No need to miss the party. Or drive her yourself and meet them there.

My DC are teens now but both used booster seats longer than their peers, until around 11. However, there were occasions where they got a lift etc without one because of circumstances. As they got older, especially, I wouldn't have had them missing out on things because of a short journey without a car seat, although I always made them have one for motorway journeys etc. As a parent, you use your judgement.

How strange. We all would do musical car seats if a parent was giving our kids lifts. I certainly didn't have three spare car seats to give lifts, I expected the other parent to transfer theirs into the car.

We give and receive lifts for school things frequently and just give our own car seats. We do have six though (twins + having a nanny).

They should be over 135cm to travel without a booster seat these days.

DS1 is in year 5 and I'd say almost all of the class is taller than that (so very handy for giving lifts).

When he was younger and we'd get or give lifts we'd usually give the other parent our booster seat. When we'd give lifts we would put the other DC in DS2s booster seat (they were both the same age rating, we'd just adjust the headrest bit to make it a bit taller).

Maybe I'm out of touch but mine stopped having child car seats when they were about 3 and moved onto booster seats.

At 7 they were just strapped in with a normal seat belt.

Is that not the norm nowadays?

At 7 mine were in a booster seat. So we just used to hand that over with the child. I don’t think you should stop your DD going though or worry about what other parents do with their children.

I don't think it is. My DD is 7 and she is 136cm
I would send a booster seat (not hbb) to school in this circumstance. I think you'll find the other mums might be dropping seats off in advance

Dd2 is 7, and I wouldn't dream about transporting her or her friends without at least a simple booster seat (no back) I have 2 in the boot for unexpected journeys. DD1 is very tall and came out of a car seat at 8, but i used one without of her friends until she was 9.5 as she isn't very tall, even though her parents weren't. If they are in my car they are my responsibility

If a child is over the height required then the other parent is not doing anything wrong if they dont use them, parents can still use them if they want no one is stopping them

DS is a short 9, we‘re not in the UK. Whenever we car pool with other parents, we always discuss who will provide the car seat. Most families have extra booster seats either at home or in the boot. A rural area, most of us also have proper seats for the necessary 2nd car. We had a stage of about 2 years where we‘d provide a booster seat for trips in other people’s cars but he had a super duper seat in our own cars.

Solutions can be creative (he hated travelling to sports club with a tall classmate as the family would give him the little sister‘s bright pink seat), but we‘ve never shared lifts without ensuring all children have an appropriate seat.

My 7 year old is in a high back booster and will be for a long time. I’m happy for her to have lifts with a booster- would not be happy for her to be without a car seat at all. She’s tiny.

I also don’t believe that many 7 year olds have outgrown the height requirements to the extent they shouldn’t be in a seat at all. My 9 year old now has by a good 10cm and now doesn’t use one but she’s exceptionally tall and 91st centile and fits the seatbelt fine with feet firmly on the floor. She was definitely in a booster seat at 7, probably still a high back and I’d always keep a booster in the car as some of her friends aren’t the legal height to not have one.

My DD is 9 and still in a high backed car seat as she is under the height requirement and the non-high backed booster type seats are dangerous and not recommended at all. I’ve always offered to give DD’s car seat if lifts are offered. I’m surprised by lax attitudes to car seats.

That is particularly small for a 14 year old though (4'9"?) My teens were about a foot taller by then.

It's not, 98th centile is 132cm for 7 year olds, people overestimate height

Wow your boys must be huge DS is 7 one of the tallest 2/3 in his class, he's 98th centile at 132cm and still under the height limit for a car seat

I've offered lifts to a party for my 7yo's friends and they put their car seat in my car when I pick up. Just take your car seat to the car when you take your DC to the car.

I know a family who all suffered life altering injuries from a head on collision from a driver who’d had a heart attack at the wheel. Nothing they could do, and being a “good driver” was utterly irrelevant. I’ve been rear ended when stationary by a van whose driver was on his phone - and I was in a dual control learner car! Again, not my own fault.

My DD was in one until she was a very tall 11, then I let her off because the seatbelt sat low enough to be safe. DS is a well grown 11 but he’s keeping his booster for a while yet because the belt still cuts across his throat. I don’t want him garotted the next time some dickhead thinks phoning his wife us more important than looking where he’s going!

Both of mine were annoyed at growing out of their high back though. Much easier to sleep in.

Id be a party pooper to keep a LO safe. And I would only drive my kid without one in an emergency - like a someones bleeding and we need a lift to a hospital emergency

I do know a couple of women who are not that tall. In fact I have a friend who has a booster cushion for when she is driving.
OP, In the UK, I think a lot of people have a couple of spare booster seats which are legal at 7 unless your child is very small.

You drop child and car seat to the house of the person driving? That’s normal.

just today I drove past some children heading out to a kids party. One waiting for other one further up the road being walked to the house with mum carrying the car seat.