We just went to WDW, and my kids are turning 3 and 5 this summer. They definitely prefer MGM over AK. They loved Disney Playhouse, Beauty and the Beast, the new Stunt Show, Great Movie Ride, Muppets 3-D, the HISTK Playground, and Pizza Planet. It's also the best park to meet characters. There is a line at Al's Toy Barn for Woody, Buzz, and Jessie, but there are several other character opportunities with little to no wait. The parade is lots of fun and a chance to see many characters up close. They also may enjoy Fantasmic. Your 5 year old can go on Star Tours and Tower of Terror, but the latter may be too frightening.
We found very little for our kids to do at AK. There's the safari, but it's hit or miss if you actually get to see any animals up close. The Festival of the Lion King show is good, but my BOYS like the Beauty and the Beast show much better. It's a Bug's Life is too scary for my kids, as is the Dinosaur ride. The Boneyard was fun, but it is basically just a big sandbox and playground. Triceratops Spin is a fun ride, but it's basically the same thing as Dumbo or the Magic Carpets in MK. They were too small for the Primevel Whirl ride and I think also for the water ride. Plus it seems to take a long time to get to and from AK, and it's so much hotter than the other parks.