When do backless booster cushions (in cars) become OK?

[Smithagain]

Any car seat experts out there?

I'm finding it hard to locate chapter and verse about when booster cushions, without a back, become an "appropriate child restraint."

DD1 is 5.5 and most of her friends now use boosters without a back. She's still in a high-back booster. She is small for her age and I prefer that she carries on using it, but there are occasions when she need to travel in someone else's car and I don't know how long I need to carry on making special arrangements to make sure she has her own seat.

I keep trying to work it out from the various web sites, but they make my brain spin!

So the way to go is "yes I know a booster cushion may be legally OK, but I believe she needs the additional protection from her high-backed booster - particularly as she is small and light for her age and the seat belt gives her little side impact protection."

... and ignore comments from friends who feel guilty that they haven't checked their own booster's suitability for their child ....... (ducks and runs)

both my older dd's are in still in high back boosters. they are 8 and 5. 8year old still has plenty of room in hers, she has a light britax one (kid or highliner, cant remember which?) which is pretty easy to move between cars.

My Ds1 is 10 and still in a high backed booster seat (britax hi-liner) and he will be in it for a while too, as he is only 123cm tall and weighes about 3st 8.

I'm also not keen on the cushions.

My 9 year old dd1 is less than 3.5 stone (still 2 or 3 pounds less) she's 142cm tall though, so legally doesn't need a seat at all

she normally uses a britax highliner though I'm trying to fatten her up - buying clothes is tricky - trousers just fall down!