I think you'll find that everyone has different "must-do"s. Considering the ages of your kids I'd say the must-do's for them would be:
MGM: RocknRoller Coaster, Tower of Terror, Star Tours, Who Wants to be a Millionaire--Play it
AK: Safari, Dinosaur, Primeval Whirl, Tough to be a Bug
Epcot: Test Track, Honey I Shrunk the Audience
MK: Space Mountain, Buzz, Splash Mountain, Big Thunder Mountain Railroad, Haunted Mansion
In addition, some other things that I always recommend are:
MGM: Fantasmic, Beauty and the Beast
AK: Festival of the Lion King
Epcot: touring all the countries...probably not that interesting to the kids, but I think it's a neat way to learn about other cultures, and mom and dad need to do the things they want too!, Spaceship Earth
MK: Pirates
I think the best thing to do is make a list of all rides/attractions by park, give each member of the group a copy of the list and have them mark down their must do's and would like to do if time, and then compare. Plan to hit everyone's must do's, and as many of the would like to's as you can.