Yeah ours do the same. We had to buy two seats, not cheap when you have two young children at the same time. The 8 year old has only just become tall enough to not need the booster any more so we have had good value out of those seats.
Oddly enough, despite spending the last year complaining that it was unfair she had to use the booster while her elder sisters didn't, she has now asked for the booster to be put back in because she can't see out of the back window in the Corsa because the rear seat sits so low!!
She can legally still use it for a few more months yet as she is below the weight limit.