Various ways of doing it , but everything in one panel Van is not really going to work I'm afraid.Another vote for van plus caravan.
VW Caravelle, caravan and awning is an absolute winner for large families.
We camp with two families of seven, both have this set-up.
It works very well for them, I'm quiet envious on occasion.
They've got Cadacs large enough to cook for eight, room for kids toys, large awnings so that adults and kids all have space.
Leaving the caravan on site offers so much freedom to go exploring or even just popping to Tesco.
You'll rip through food supplies in the van. Fridge refill every third or fourth day?
Where will the kid's car seats, baby buggy, table and chairs live?
Think about clothes, shoes, towels, dirty washing, crockery and cooking utensils, even the number of loo rolls you'll need to carry.
Awnings are heavy and take up a lot of space, you'll probably need two toilet cassettes too.
Will the kids want to take bikes/scooters/skateboards and so on?
You need to keep everyone happy otherwise it's no fun, think about the practicalities of a panel van....
Personally I'd go bus and awning, but your idea is probably just as good.
But if you are going that way I would think a huge caravan and a normal eight seater car would be better than a motorhome altogether.
When not on holiday the car would be more useful than a motorhome.