When we go for the 1st time ds will be 7 and dd will be 3 1/2. DS uses a booster w/no back, so I thought I'd stow it in a carry on bag, hopefully! DD uses a high back booster, 5 strap harness type, but she could use one with no back with the seat belt. I'm wondering if we should spend $15. to buy her one with no back, just for the trip. Or should we just use the highback one on the plane? I think the no-back ones would each fit in a seperate carry-on bags. Anyone ever done this before? It would save us $16. per day from the car rental place.