As CM Dad says - Put you & mom & kids in one room, and husband & dad in other room. That way only your husband and MIL would need to get blank key cards (good for room entrance only) and switch rooms once onboard.
OK, room 1 , you, MIL, and one kid.
Room 2, husband, FIL
Room 3 one kid (of yours) and whoever else it is .
If there are extra kids, spread them out in the various rooms.
All 3 rooms get silver status.
ALSO: you should call, not do the booking online. You're more likely to get a CM who can work with you to sort the people out.