If you want to restrict your kids from some stuff on the internet, here's a guide for Tinyproxy on Ubuntu.
It's found at the Nuxified forum
At my house, I'm a dad who uses Linux. I have some kids. They're starting to grow up and go to more questionable sites on the Internet (on urging from friends) and it's my job to police it. I am one of these dads who think that kids get enough exposure to bad stuff from TV, movies, and the public schools, and I don't think they need any more influences until their minds are mature enough to handle it. I also limit the kinds of movies they watch and the kinds of TV shows too. With my oldest child, I have let her watch a bit more than her younger brother, so I mean to say that I'm not the meanest dad in the world. I really do let a kid grow up — I just try to wait until I think their minds are mature enough to wrap around certain concepts and strong enough to fend off peer pressure.