Restricting User Access Hours

I'm not sure where to post this, so have posted it a few places.

As a parent, I would like my children to not be able to use the computers
during certain hours of the night. There are two computers: one operates on
XP; the other on Vista

I was looking through the User Account and Computer Management console to
see if there was a way to establish user or group policies that restricted
particular users or groups from having the ability to log on during certain
hours of the day. However, I had no success finding anything. Is there any
way to do this that I was not able to find? Or, would I need a separate
program to be able to make these restrictions happen?

Thanks,
Zakhary