I have been to both land and world. Living 7 miles from the world, i am biased. While I would suggest waiting til the magic age of 5, you can still have a good vacation in MCO with very young children. My main suggestion is not trying to do everything in one trip.... Plan to rest in the afternoon. I see way too many exhausted and cranky families who tried to go around the world in three days or less.
Have fun!