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 in Suspended Development
Stable: Currently at Version 184.108.40.206
Development: None at This Time