Hi Keema!
For your child, I would say between the ages of 4-7 is the perfect time to bring them for that magical Disney experience (although for me, "The Big Kid," it's still a blast even in the mid-30's range-smile)...
To me, 4-7 is that time when they clearly "get" where they are, they should be out of the diaper/pacifier stage, but still not too grown to be awe-inspired by Mickey and the other characters. They actually want to take pictures with the characters, and are normally tall enough to get on most of the rides with height restrictions..
My daughter, now 6, has visited at 3, 5, and 6. Although she still recalls some of the things she did when we visited at 3, 5 seemed to be where it all "hit" her. Watching her meet Minnie, Cinderella, etc., and seeing the excitement in her eyes, captures the essence of magic that I feel Disney tries to provide to everyone.
Any age you can't go wrong, but 4-7, at least to me, seems like the perfect time for a kid to visit Disney World.
Best Regards!