Gnome Nanny is an easy way to control what your kids are doing on the computer. You can limit how much time a day each one of them is browsing the web, chatting or doing email. You can also decide at which times of the day the can do this things.
To install Gnome Nanny open your Terminal window (Applications -> Accessories -> Terminal) and copy+paste the following lines:
sudo add-apt-repository ppa:nanny
sudo apt-get update && sudo apt-get install nanny
When installed you can find Nanny under System -> Administration -> Parental Control
Gnome Nanny Homepage