My 3-yo handles KDE 12.04 just fine. She can run her educational program (GCompris) with ease, including games that are really suited more to older kids, can handle finding her favorite videos in the file system, and even (sort of) works with Celestia. She needs help with the password, and with finding the Youtube videos she likes, but I or her mother can help with the typing, and she’s always supervised anyway.
Come on...get your kid a BB gun, bicycle or a football.