Where the Magic Kingdom, I think, is more geared toward the littles, Animal Kingdom is great fun for any age, especially the animal lovers. It's fun for the younger ones, with some decent rides, but lots to do and see if you're an old fart, like me, who doesn't do the rides so much anymore. Go here
What Are the Best Times to Visit Disney World? for helpful info on when and when not to go. I've used this many times with great results. You DO NOT want to go during a heavy crowd period.