Visit
www.dansguardian.org and look at their install notes.
You'll need to install that and Squid.
DansGuardian sits between the browsers and the internet, filtering eveything from ip numbers to names to attachments to words in pages.
A very complete product.
Daniel Barron has also made a commercial version for Smoothwall, but the original hack is still floating around.
Maybe some money for the good work is appropriate here...
Regards,
Peter