Our desk top has that option in the programming. It's windows Xp. Search the programs and find it. Our computer gives them a 15 minute warning then they get a screen when time is up. It gives them the option of more time in a drop down box. An adult has to enter a code and select the additional minutes for the computer to come back on their log in name.
Once their time is done for that time period they can't get back on until they have scheduled time.
For instance one of the kids can be on the computer from 10am to noon, the other one can be on from 1-3, then the first one can get on again at 4-6 and the other one from 6-8.
They cannot get on when it's not time for their own log in to be on. They don't know each others passwords and they do NOT know the adult password to get more time.
IF we give them more time it's because they did extra chores and earned it.