Limiting Kids' Computer Time?

Bibu asks: "As a parent of three, I have to spend a lot of effort to keep my kids away from the computer. Until now, we had a Linux box in which a little cron script would just shutdown the machine after half an hour, when the kids were using it. Does someone on Slashdot have a fancier solution? One that keeps track, controls the total time per user per day, and would warn the user of the upcoming deadline (e.g. in five minutes their time is up)? Since we just moved to Mac OS X, solutions for that platform are preferred."