With Watcher 5, setup a curfew for week days and week ends. This way, your kids won't use the computer too early or too late !
Decide how long a given user can use the computer on a regular basis (week days or week ends), and let Watcher handle everyting.
As soon as you have configured Watcher, every computer in your network will be aware of the change.
How long is your son using the computer every day? Watcher lets you know by providing detailed statistics.