There's something for little ones at every park. DS has loved Epcot since we first took him at 15 months because he digs the bands and live entertainment. He has danced to drums in Japan, been the center of attention of a crowd in France when the performer put balloon bug antennae on him, bopped til he almost passed out to the Beatles cover band in England, and wiggled to the fiddlers in Canada.
We've had great days in AK and MGM as well; it is all in what you are looking for. As a family, we enjoy wandering through World Showcase probably more than anything, though.