My kids are 11, 10,10,6 and 4. We spend more time at Epcot than MK. For us AK, and MGM are each less than a day, with us going back to Epcot at night. I find that if we do a few countries at a time, the kids really get into them. Plus they love the rides, aquarium, and all the hands on computer stuff. it amazes me how many people just walk right through the Innovations (sp?) without really looking at the exhibits.
I think Epcot is the best park at WDW.