Laura:
I have a five-year-old and a three-year-old and have been traveling to Walt Disney World with them since they were infants. While we have fun at all of the four theme parks, the one park we never can go wrong with is
Magic Kingdom.
While only a portion of the many Magic Kingdom attractions, our family always enjoys Dumbo the Flying Elephant, it's a small world, Jungle Cruise, Mickey's PhilharMagic, Peter Pan's Flight, The Many Adventures of Winnie the Pooh, Pirate's of the Caribbean, Tomorrowland Transit Authority People Mover, Haunted Mansion, and Monster's, Inc. Laugh Floor. Your older ones, if tall enough and brave enough, may be interested to check out Splash Mountain and Big Thunder Mountain Railroad.
Enjoy your trip.