BoostApak Green

There are a few brands of foldable car seats, Maxi Cosi has one that's good from 15mo. They are expensive but honestly the price of getting the crappy one from the car rental place is about the same. Once you get too booster seat age, then the MiFold and the Boostapaks are good options

It can be cheaper to actually buy one though. We had a Trunkie to travel to France years ago. Granddaughter packed it with a mini dvd player to use on the plane. Also comfier for kids than a plane seat.

For a 7 year old I'd get a trunki boostapak, and for the 3 year old a Joie Tilt if they meet the size criteria... cheaper than hiring and you know they've not been in a crash already and can use them again.