I would reserve a separate studio for the couple that may or may not be coming and ask for a handicap shower of the type he would prefer, either a roll in shower or a tub with handrails. Gives them some privacy and the ability to have a sleepover with the grandkid(s).
Then for the other 6 six, it would depend on if you all would be comfortable sharing a 1 BR, a 2BR or would you prefer two studios. You didn't give the age of either child but does the older child need naps? Can the infant sleep while traveling through the parks and are the parents both okay with that? If kids need naps I would vote for the 2 BR or two studios.
You might want to check with the disabilities thread about what resort would work for your FIL. I think that the resort I would have doubts about is OKW. If your FIL is using a scooter he will need to have an elevator in the building (only three buildings have elevators, Building 62, 63 and 64 all on Miller's road) or you need to have a room on the first floor. DVC tries to be accommodating especially for handicapped guests but the handicap reservations are not a guaranteed reservation and first floor rooms might not be available. At least with all of the other DVC resorts you will have access to an elevator.
I would pick a resort that your guests would enjoy and perhaps pick one that is close to the parks they are interested in. Or pick the resort they would love. Would the kids really enjoy seeing the animals at AKV and would the adults enjoy watching the kids? Or is going to a park more important? Would a resort near a park that the parents with kids like going will work out to be good for your FIL who could return to the resort easily when he needs a rest or wants a break from the parks, but doesn't want the family to have to drive him back?
I would consider BLT, or BWV or BCV for location near parks. These resorts would make it easier for your FIL to use his scooter to get back to the room from some of the parks.
