My 3 1/2 year old did the Muppet 3 D movie and really didn't "get it." And he wasn't the only little kid in the theatre that didn't.
With these types of attractions, I think part of how your kid will react depends on how good they are with regular movies. If they sit quietly in a regular theatre, they will sit quietly here. If they find regular movies boring, these may not be different. If they find regular movies too intense, these will be intense. I have one girlfriend whose kid couldn't go to any movie until he was seven - they were all too intense and he had to watch movies at home - another who has been taking their children to see movies like Spiderman since toddlerhood without a problem.
My advice is start in Fantasyland at MK with the small rides - Not Snow White! Dumbo, then Small World, then Pooh, then Peter Pan. If those have been handled with no issues, move up to Snow White. Still no issues, try the Haunted Mansion/Pirates of the Carribean. Still no issues, when you move parks you can try the scarier stuff, or try the thrill type rides that she is tall enough for in the MK. You may know quickly - as she pulls you toward Haunted Mansion saying "Small World is Boring!"