I second every one else here. Our daughters just love it, and they were 4 and 18 months the first time we went.
My advice if it's your first trip, though. Save what you think they'll like the best (usually MK, but actually now Epcot for our kids) for last. Everything at WDW is great - kids will almost always find something to enjoy. And if you save the biggest winner for last, it all builds up and everything is better than the thing before. But if they start with the fantasy rides in MK and then try to do some of the more "educational" things, they may have been expecting a lot more Fantasyland, and be a bit disappointed. This is what we did the first trip, and it worked great. MK was a tremendous end to a tremendous trip.
My advice if it's your first trip, though. Save what you think they'll like the best (usually MK, but actually now Epcot for our kids) for last. Everything at WDW is great - kids will almost always find something to enjoy. And if you save the biggest winner for last, it all builds up and everything is better than the thing before. But if they start with the fantasy rides in MK and then try to do some of the more "educational" things, they may have been expecting a lot more Fantasyland, and be a bit disappointed. This is what we did the first trip, and it worked great. MK was a tremendous end to a tremendous trip.