Something to be said for all of them. I'd say MGM and Epcot were more popular with both 2 and 4 year old boys. AK has a great playground (Dig Site) but he didn't like the Africa or Asia trails or Conservation Station.
MGM: Cars Stunt Show, Muppets, Great Movie Ride, Playground, Star Tours (4 yr old only).
Epcot: Innoventions (they love some of the exhibits), Journey into Imagination (and play area there), Turtle Talk, Test Track (4 yr old only), Maelstrom, toy trains at German, Japanese rock garden, English garden.
Basically anyplace they can run around (especially with other kids) and raise hell they will enjoy.