I just visited with my 3 year old (almost 4) and 1 year old. We stayed for a week and went to each park. Things they liked:
Animal Kingdom - I thought this one would be a big hit since we all love animals and our local zoo, but they were bored (baby even fell asleep) on the Safari and the shows weren't much better. The Boneyard was the highlight of their morning sadly. Tusker House breakfast was okay - food was great, characters very rushed.
DHS - The Disney Junior show was a BIG hit - both kids have watched the Clubhouse and the shows and they love, love, loved this one. Right outside were characters and my older kid was all over them. The Muppet Vision 3D was actually a big hit with both too (the baby wore the 3d glasses for almost the entire show, 3 year old giggled at all the right places). The fountain outside Muppet Vision - also a big hit.

The Frozen sing a long was actually pretty fun, too. Toy Story was fun for both (3 yr old to ride, baby to see). I was really surprised at how good of a time the kids had here. We didn't make it to Hollywood and Vine but that is a stellar character meal. We ate at 50s Prime Time and the kids had fun.
Epcot - Nemo ride was fun, but Turtle Talk was a big disappointment. Spaceship Earth was mildly interesting to the 3 yr old. They weren't really into the aquariums like I thought they would be either. Lines were bad so we missed things like Figment. Good character meets though. We didn't even make it to the World Showcase before naptime hit.
I'd vote HS first and Epcot second. I love AK but the kids really did not.