Now that my 4 year old is learning to read and type, I'd like to set up some form of parental controls on my network. I consider myself a moderately competant Linux user, but I do not have anything approaching the skills of the talented sysadmins I've worked with over the years. So, I'd like to reach out to the Slashdot community for advice on how to lock down my network so that they won't accidentally bump into the seedier side of the web.
What are the best firewall apps to use for internal filtering? Do (or could) any of them support pro-active whitelisting, where I could get an email with a blocked URL that my kids tried to visit, and in response — I set up a rule to allow or deny access to the URL. This is a fairly mainstream feature of various parental control software out there, how could a similar solution be set up in Linux?
I'm going to assume that my kids are smarter than me and will one day figure out how to own the network. You get bonus points if you can give pointers on setting up a dual system, one that they can hack, and one that is running in the background in silent or stealth mode to monitor activity. My intent here is not to be big brother, but rather a responsible parent.