I would stick with Disney parks. Would it be possible for you to get a three-day hopper ticket and do more than two parks since you're going to be there three days? You can't get them through Disney, but they are available -- someone else will be able to tell you where. If you had hopper tickets, you could go to all four parks if you wanted. You also would qualify for e-night if any end up being offered. These are nights that Mk is open for three extra hours for $12 to a limited number of guests. You have to be a resort guest with a multi-day ticket to qualify. Nine rides are open, and, because there's no lines, you usually can ride your favorites multiple times.
MK is my 9-year-old's favorite park. He loves all three mountains and Buzz Lightyear. He feels he's way past Fantasyland -- all of it -- probably because of the influence of two older brothers. If he could only do two parks, he would have trouble deciding betwen Epcot and MGM. At Epcot, he likes Test Track, Mission Space Imagination and (even though it's just a small thing) Ice Station Cool. He also really likes World Showcase, although this has been a gradual development. He loves Imaginum (living statues) in Italy, Miyuki in Japan and Le Mime Roland and A Chair Act in France and to eat at San Angel, Marrakesh and Teppanyaki. At MGM, his favorites are Rock 'n Roller Coaster, Star Tours and Muppets.