DD2 has a half booster that we use on long trips, but not every day. She's quite tall & the seat slides out from under her all the time.

I've bought DD3 a seat that goes up to 8 years rather than the standard 7 & for her age she's short compared to my other kids, so she might still be in it at 9.

I know all kids DD2's age (7) have different rules in their car. Some have a booster, some don't. But the ones that do, just do. That's the rule in their car, & there's no choice.