The "Teach Me Time!" alarm clock has a very clever design. As well as the analogue and digital display helping your little one learn how to tell the time, it also teaches them when they are allowed to get up, hopefully saving your precious early morning sleep.
As well as built-in time themed games for kiddies, there is a "secret" control panel where you can set the "OK to Wake" alarm. This will cause the clock to glow yellow during the night and turn green when the kids are allowed to get out of bed.
So even if your child is to young to actually tell the time, they will know (or so the theory goes) that unless the clock is emanating that green glow, they have to stay put. $40, available now from ThinkGeek.