A new version of KChildlock v0.57.1 has been released. KChildlock is a tool to monitor and restrict the time a children spends on the computer. The limits can be specified per day of the week, by lower and upper hour limits, maximum daily usage time, and maximum weekly usage time.
The same restriction limits can be applied to applications based on the user login. It requires the KDE4 Desktop.
Here are some screenshots of KChildlock.

You can download the least version of KChildlock from here.