Paul is 3 and has been to Disney World four times.
Obviously MK is number 1 at that age, but we've found that both Epcot and AK offer WAY more for our little guy than MGM. That might just be us, though . . .
At AK he loves the Festival of the Lion King, Flights of Wonder, Pochanahotas and Her Forest Friends, the Safari, the Gorilla and Tiger walking trails, Triceratops Spin, the Boneyard, It's Tough To Be A Bug (this one scares some kids, but he loves it), the petting area at Conservation Station, just exploring the Oasis, and more.
At Epcot, he loves all the entertainment in World Showcase (especially the Mo Rockin' band, the Japanese Drummers, and the Chinese acrobats), the films in all the countries - especially China, the Maelstrom ride in Norway, Spaceship Earth, Journey Into Your Imagination, all of the Living Seas - especially Turtle Talk, the dancing Fountain of Nations (one of his favorite things in all of Disney World), and lots, lots more. Oh, and he is crazy for Illuminations! It's hands-down one of his very favorite things at Disney.
MGM is by far the park that offers the least for him. We don't watch Playhouse Disney, so he doesn't really care about that show. He does like the Little Mermaid show. Other than that, there's not a lot there for the littlle ones, it seems to us. He does love Fantasmic, though.