I tell you something that is scary these days, the amount of content that is available to kids whilst they are in school. My stepdaughter goes to a secondary school that's been designated as a 'Technology College', which in my mind
should make them better equiped to a: monitor where kids are accessing, and b: block access to those sites. Yet when she comes home and says to us, oh I was looking at that earlier in school, it makes me wonder what precautions they
do take.
I'm quite lucky in that I have a fairly good understanding of things, and for the most part over the last 5 or 6 years my daughters internet access has been controlled by a combination of an opensource program called Smoothwall (which turns an old pc into your internet router). This uses a piece of proxy software called Squid - all web traffic gets transparently directed through it - this in conjuction with a list of allowed sites, which has been built up over time, and everything else is blocked. When she first started out on the internet, it was a case of, everything is blocked, unless she provided us with the website address, so we could take a look and approve / dis-aprove, sure, it may seem restrictive, but given the vast amount of content out there these days..... This was also useful in stopping website adverts/popups, as the advert couldn't load as traffic wasn't allowed to the website (great all round haha). As she's started getting older, it still goes through the proxy, though it's a case of she has enough rope, just hopefully she won't hang herself as it's monitored from time to time, and at any point we can monitor/react to exactly what she is doing through the use of remote access tools. It's also capable of logging msn conversations, and IRC conversations, and is well worth the look for someone that wants to look into a project. It's also great i nthe fact that the internet can be halted for that specific machine, but without affecting the others on the network.
There are commercial programs out there such as net nanny mentioned above, which really use the same kind of principles, my brother-in-law used to swear by
cybersitter, which has 14 years experience in the market.
Surf Control is another which is owned by websense (who make enterprise level filtering programs for the workplace).