MK- Fantasy Land is always a kid's (and some adults) favorites. It really depends on what they like, they might like Big THunder Railroad and Splash Mountain (my nephew was 3 1/2 last year when he went on both and loved them!) The Haunted House might be too scary. Don't miss Buzz LIghtyear's Space Ranger Spin in Tomorrowland, and Mickey's Toontown Fair.
AK- I'm not the biggest fan of this park. You could skip Dinosaur, that's probably too scary.
MGM- You could probably skip Tower of Terror and the Aerosmith roller coaster. The Great Movie ride is kind of boring for little kids also.
EPCOT- Skip Mission:Space, they may or may not like Test Track, again my nephew loved it. Journey to Imagination used to be one of my fave's as a kid, but they changed it... Lots of slow moving rides here.