Since 4 of them are kids, I think you could fit 12 people. It would be tight. There are safari buses that run to the beaches all day. They sit at ARH. The last time I used one it was $6 per person each way, but that has been a long time ago. May have changed.

The roads on Anegada are very flat and not many cars. In area's where the road isn't finished they can be very bumpy, just go slow and you will be fine. Put the kids up towards the front of the truck and they will be fine.