I think that kids will find a way around most any parental controls that are in place. Really you need to talk with your kid/s and explain to them why they shouldnt go to certain sites, etc.
For Example, at my school they use an advanced Content blocker on all machines. Half the time it blocks the content used for Science studies and Calculus, but then the person next to me searches britney spears and is pulling up porn sites
and if you really want to get to a site, you go through google and use the google cache function....works for bringing up text although sometimes pictures still get blocked
there is no perfect content blocker out there yet in my mind....except for talking with your kid/s