I feel that I was not.
My parents and my neice are coming to visit for 10 days over the Easterbreak. They are flying, we live on the continent and they are in the UK. I phoned them to remind them of little things they would need, like swimming stuff for DN, her pencil grip, her camera and her booster seat. They told me they were not planning to bring one. DN is 7.8 and around 3'9". I told them that 1. We were not going to risk being on the wrong side of the law, and 2. I was not willing to see DN get horrifically injurued in case there is an accident because she had no booster seat.
I can't believe they were willing to risk it and would just have turned up at the airport with nothing. My family think I'm being overly concerned, and that it's an awkward item to bring.
I told them I would buy on myself here but they agreed to bring their one with.
I don't have older children so I don't know how vital they are, but I really feel that IWNBU.