We just used a regular digital clock and told DS that he wasn't allowed to make noise or go out of his room until after 6am. We started this when he was 2 or 3. The first morning he woke us at 5:26 and we thought we would have to tape over the bottom numbers but he didn't want that and he never messed it up again.

It didn't help him get any more sleep but at least the rest of the house did smile He's now 10 and most mornings he's reading in his room when I get up at 6/6:30.