Yeah, your 7yo could totally put her booster in the back. My 8.5yo moves his booster all over this truck!
When I had 2 little ones and then had the 3rd, it was hard to configure the carseats in the mini-van - but with just the one now and a portable seat at that (not latched in) - it's great
I don't know your carseat laws but ours are 8yr *or* 59lb/59" - we're waiting for the height at this point and then he's DONE!