Re: Families of 7 - what do you drive?
We have a grand caravan as well - and I sure hope it'll fit us all in the spring!
My kids are older than yours, so car seats are much different for us I'm sure. We will (likely, depending on what fits best where) have DD (in seat belt), DS3 (Radian, will be FF at that point) in the rear row, and DS1 or 2 (both boosters). Then DS1 or 2 (whoever isn't in the rear) (again, booster) in one middle seat, Babe (infant seat, likely) in the other middle seat.
With younger kids, and thus more carseats, it would be a matter of puzzling and seeing what would fit next to what, kwim? A couple Radians might be necessary, possibly 3, for the rear row - but even if you had to buy 3 new seats, that's going to be cheaper than a new vehicle, kwim?