Romaco Timeout is a free computer tool that can restrict the amount of time that a child (or anyone) uses their computer for. It has modules that can restrict the length of a session, create a cumulative daily time limit, set a window of time that the computer is available for, as well as monitoring web usage. It also has website and program blocking capabilities.
Timeout is Discontinued
See the News
page for more details
Stable: Currently at Version 188.8.131.52