elfs | IpTables Gurus out there?

I have a rather ordinary set-up at home: A mini-tower box running Linux acts as my firewall, using NAT to assist everyone inside the house from getting out but preventing anyone from getting in. It's not perfect, but it's your standard solid setup. I haven't had any problem with it and I keep it updated.

I would like to run Dan's Guardian on the NAT box, but I would like only some of the machines in the house to be routed through it, namely, the kids'. Dan's Guardian is basically a proxy that uses a cache (Squid, in this case) to retrieve content from the web, and then analyzes both the addresses and the content for things you might not want.

Can anybody tell me what the iptables magic is for directing traffic from a specific host on a specific subnet to the proxy engine? The idea here is to prevent anything that tries to "route around" the proxy (like, the kids figure out how to turn "use proxy" off on their browsers) from being able to go anywhere without going through it.