I've taken kids 3-7 into both parks.
In IoA they loved Suess Landing, and thought that was the whole park. They didn't want to leave. My advice - don't. Let them enjoy themselves rather than force-march them to see more. That'll just overwhelm them, they'll get tired and cranky and enjoy what they do see less. Younger kids are easier to please. If they get their fill, head for toon lagoon and stop my marvel island for a pic with spider-man on the way. If they still have energy, head for Camp Jurasic and let them run, run, run.
In USF they have E.T., barney, woody woodpecker and curious george. My young kids loved curious george; to a young, small person it's like a whole waterpark to them - and it is, they couldn't go on giant slides anyway. Our kids loved the giant bucket that dumped every few minutes. We were prepared and kids had bathing suits and we just let 'em go. The place is small enough its easy to track them, with only 1 way in and out. Despicable Me will probably be a hoot, haven't seen it but or kids loved the jimmy neutron it replaced (same ride mechanism).
Either is good, just don't push the kids too hard, let them go at their own pace. Better to have them enjoy some than have a meltdown trying to see it all.