I would visit all of them, but with shorter days at Animal Kingdom and MGM.
At 3, Magic Kingdom was my dd's favorite, with Epcot a close second. There is a lot to see in Epcot and for much of it you don't have to be able to go on a 'scary' ride. She LOVES Spaceship Earth, The Land, the Living Seas, all of World Showcase, loads of characters, and shows, people dancing/performing in the streets etc. She really really loved the parade in Epcot, but it's not on any more. We spend a lot of time in Epcot, and she always seems to enjoy it. I think it's my favorite park, so maybe my enthusiasm rubs off on her.
MGM - Playhouse Disney is a must see, but to be honest, there's not a lot else that was ok for my dd. She made it through Muppets, but I wouldn't say she enjoyed it. She hated the Little Mermaid, the scene with Ursula is very intense. We haven't tried the Great Movie Ride, but I really don't think she would like it.
She did like Beauty and the Beast at 4, not sure if she would have sat through it at 2 though
Saying that, we always go for the day, and she is quite happy to look around and we usually meet some characters, but there are not a lot of suitable 'attractions'. We did enjoy the parade.
Animal Kingdom - She really loves the Lion King Show. Really Really loves it. She also loves the Pocahontas show, but Pocahontas is her all time favorite character. We do the safari, and teh 2 walks. She kinda liked the Safarai, although was a little unsure about the poacher story line. She also liked the train ride to the conservation station.
Bev