For parental control in Linux, the best available tool is "timekpr". This can be installed in debian linux versions like Mint, Ubuntu etc.
Here is the link to the website to download the latest version:
https://launchpad.net/timekpr
Click All downloads and you can find different versions. I personally installed the .deb version and it works excellent. It comes with a GUI, where you can set the day, time and allowable time limits for every user in the system.
