Just to throw in my 2 cents.................don't do EMH mornings at MGM.............the kid stuff isn't open, and my ds and I just hung around in RnRC courtyard waiting for the big boys to do the big rides.
My son was 2, and loved Playhouse Disney and Little Mermaid. Knowing him as I do, he will love this time at 4, Indiana Jones, Star Tours and the HISTK playground as well (wasn't impressed with it last time).
However, he was CRAZY about Animal Kingdom. Just nuts about it. Loved the Safari, the dinosaur statues near the boneyard and the boneyard, LOVED the animal trails, the happy meal he got at Restaurantosaurus, the ducks he watched at Flame Tree barbecue. It was his favorite park of the entire trip, even above Magic Kingdom. And now, we'll be going at his 4th birthday, and it's where he wants to spend his birthday. We didn't even go to Conservation Station that trip, and our 6 year old son had loved that on a previous trip, so it's going to be a surprise for him this time (they have educational exhibits and a petting zoo).
I love the suggestion of EMH at AK, and then hopping to MGM for Playhouse Disney and the Mermaid..............we also love Fantasmic, though it's scary for some young kids.........we had ours "help" Mickey by "punching" the bad guys (just throwing punches in the air when bad guys showed up), and he was fine.