To ask about car seats for older kids?

[derrydarling]

My eldest is just 135cm, the minimum height for no longer needing a car seat. They are 8yo. A few friends have stopped using a car seat altogether (all around the same height, maybe 1-2cm taller), but I use a high back booster.
I think I'm about to get into a situation where I'll be giving lifts to two other DC's, one aged 6 so will def need a car seat, and one who's slightly taller than my oldest DC. The easiest thing to do would be to put the other smaller DC into my older one's booster, and the two older ones have no car seat. But is that safe?! I'd struggle to fit an extra car seat, or two , into my car. And I don't think the other mum even has one for her child anymore anyway.
But it just seems wrong that an 8yo no longer needs any kind of car seat?!

My son was 9 when I got rid of the car seat. For reference he was in 10-11 clothing so a tall boy. I felt like it was too young but it was hurting his hips, 8 does seem awfully young. I would ask the mother regarding her eldest, and let your son go sans car seat while these other children are in, and maybe pop him in it when it’s just you guys.
If I ever took the kids out in our teeny car and had a passenger the eldest had to have no car seat from 7 as there was no room. Our other car is a 7 seater so 99% of the time he was in his booster!