Epcot has a surprising amount of things kids love. We took my 5 year old niece in March and wondered if she'd be bored but she loved EVERYTHING. She loved Turtle talk with Crush (who picked her to ask a question..awwww) Soaring, and was fascinated by Ellen's Universe of Energy. I have to admit, I even loved that one. She wasn't big enough to go on Mission Space but didn't seem to care. She loved the fountains and ran through the kids sprinklers for a long time. We stayed for illuminations and the look on her face was worth a million dollars.
If your kids are into technology and video game type activities, Innoventions can keep them occupied forever. We had a hard time getting her out of there. So much to do in those pavilions.
All in all, we were glad we took her to Epcot. It was much better than expected with a small child.
We did do MGM too but the only thing she was really interested in was the Voyage of the Little Mermaid and Playhouse. She couldn't go on RnR and we were afraid to take her on ToT in case she got too frightened. She did however love Fantasmic and she still talks about it. That's a great show for kids.
Have fun on your trip. There's something for everyone at each park.