This is a little long and complicated to be called a "hint", but here's how to make sure a child can't use the Mac past bedtime.
[Editor's note: Jerry has written an amazingly thorough write-up on how he set up his Mac to force an end to "machine usage time" ... I won't even claim to have tried this one, but if you're in the need for such a function, his article looks very complete.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003021005501438