I've used squid to do it. Squid is great, but content filters, in general, 
suck. 

For squid, you can use squidblock:
http://www.hklc.com/squidblock/

And the slightly more intelignet squidguard
http://www.squidgaurd.org

But I have to agree with Scott. There is no better content filter than human 
eyes. If you don't have time to supervise children's net usage, turn off the 
computer. 

-- 
Andrew S. Zbikowski | http://www.ringworld.org
"We can learn much more from wise words, little
from wisecracks and less from wise guys."
--William Arthur Ward