Get a sunshine radian XT car seat or multiples of them. You can fit a booster and 2 sunshines in a cavilier.
http://www.amazon.ca/Sunshine-Kids-R.../dp/B001O5CM7E
It is the narrowist carseat on the market and goes frm rear facing infant to 80lbs forward facing.
On the new car front the new Chevy Traverse and a few of the other seats now are designed to access the 3rd row with car seats installed without the crawling over the seat.