Need to buy a Booster Seat for my parents car. They have a tiny backseat and DS rarely rides in it, and never on the highway (really only short distances in slow city traffic) so I'm fine with a Backless. Any recos? Easiest install and smallest size are my main priorities.

And then for my car, should I move DS (4.5, 41") to a high back booster and give DD his convertible (this would be in about 3 months),
should I buy DD a new convertible and keep DS in his Maxi Cosi Pria longer (and if so, until when?) Our Pria seat will be 4 years old this summer. The only downside to this besides cost is that I would like to be able to have a 5th passenger in my car and the middle seat will be very cramped with 2 convertibles.