An idea for any parent with kids is a hidden spy software on your pc, see what your kids are up to when your not looking. I found it very handy and caught out one of my teenagers friends telling another one to kill themselves, they were shocked when I showed them EVERYTHING they had been doing on MY pc's.
I do belive the one I use can block sites but personally it is better to monitor them than to block their use of sites like Bebo and youtube, is called Eagle Eyes