OK, so I've got 5 kids, and know a LOT of large families. Been doing this a while. I've had Suburbans, Volvo wagons, Full sized vans, and mini-vans.
They are thinking wrong.
Cheap purchase price, low maintenance costs, easy to beat on= Suburban. Not many other choices. It will cost them at the pump.
But they REALLY don't need a car that big, and will regret buying it.
They're answer is 7 passenger minivan. No question. But they may have to accept the fact that they will have to take a 2nd vehicle for the occasional times there are 2 more adults. They will also have to accept 2 center seats. Minivans don't seat 3 baby seats across the middle.
Unless they are having twins and already pregnant, they won't have 4 car seats. 3 max, probably 2. Most young parents seem to forget that the little guys tend to grow.
Even the Suburban, which has 3 seats across, can not fit 3 car seats, and if it could, you wouldn't be able to load the child into the middle one. Once they WERE loaded, no one could climb into the back seat. Unless the 5 year old is going to climb in the back window and be completely responsible for his own seatbelt, 3 carseats across the middle is OUT. A minivan with 2 seats in the center row and an aisle on the passenger side (not the center) will give the best access for the adult to load a child into a seat in the rear.
Volvo- WAAY too small.
I love Astros, but I'm not sure how I feel about them. They have a higher floor, which makes loading little kids in and out harder. They are also questionable for crash testing.
Don't forget the Mazda MPV. A lot of minivan for the price.
One other thing. Our lives got better when we realized 2 cars here and there wasn't so bad. We got rid of the Suburbans. My wife drives a minivan, and I drive whatever I want (usually a Miata). When we just need to run errands without kids, we jump in the Miata (or truck, or whatever). When the minivan isn't quite big enough, we take 2 vehicles. It's not that often.