I asked the same question previously. I got mixed results. Basically, from what i figured out from responses on this board and on others, in addition to what i found on the net, there aren't any free programs that'll do this to the extent that we both want and make us happy. Sure there are a few programs (like psshutdown) that you can use to shut down the pc remotely, but they've gotta be initiated each time by you each night. One person mentioned writing a batch file... but that's all the info i got out of that one guy. lol.
There's a few other things like using the command prompt and setting user time limits but it doesn't shut down the pc just stops them from logging on again after a certain time.
Basically, the only real good stuff i found were programs you'd have to pay for. I downloaded one of these, it has a 15 day trial and then its pretty cheap. It is really configurable and seemed to have a lot of the options i was looking for.
Good luck. I went with this program but haven't been able to use it... ended up confiscating the whole pc a week ago due to finding downloaded viruses and porn on their (kids) computer. Guess i don't need it now!