Is there any solution here that I'm not seeing? I think that even if the two older ones are in backless boosters, there isn't any way to fit a harness seat for the 4 year old back there too. But I'm used to the bulky Britex seats, and maybe there is something smaller? Or, what larger cars have people moved to? We also have a problem of not being able to fit my mom in the car with us.
I'd appreciate any advice!