Parental Controls – Different Settings for Different Users

Does anyone know of a good way to set up some sort of parental controls so that one user account is subject to them, but not another?

A couple of friends of mine use Ubuntu and have kids (7 to 10 years old) who use the family computer. They'd like to have some blocking of adult sites. They're not going to supervise all the time, which I know might be ideal, but there we go. And they're not particularly technical, so they don't want to have to run scripts to turn the parental controls on and off regularly, but they can cope with having different accounts for different people. I haven't found a great way of doing this. I am pretty technical, so I'm happy to spend some time at the command line to set it up, but then it needs to just work.

Please don't just link to DansGuardian. If your answer doesn't address the different user account aspect, I will vote it down. If you want to talk about general parental controls that apply to all users then please start a new question – I'm sure that plenty of people would be interested in it, and I'll link to it from this question.