You can book up to 4 cars since 4 passangers are travelling (Though I'm not sure if children can avail this or not).

If they can't then why not book 2 cars (one under your name and the other under you wife's). If they call you the morning of the flight tell them you want a minivan, if they say they can't then 2 cars should be fine.

I once did this in London where I had 5 suitcases for me and my wife so we booked two cars. I always do that if I'm 100% sure they can't provide me with the minivan.

I did the same in HKG where they called the same day and I told them if they can send a one minivan then that would be great which they did.