Also, be aware that filtering proxies are really just a weak band-aid and relatively easy to get around.

Blacklist proxies and content-filtering proxies are weak. Whitelist proxies, such as he's talking about using, are not so weak, although their usefulness is limited to situations where it's acceptable to block pretty much the whole internet with a few exceptions.

(I'm assuming here that he's going to run the proxy on the firewall, not on the desktop, and that the firewall will be set up to drop any unproxied traffic. Otherwise of course they'll just change the browser setting so it doesn't use the proxy.)

This wouldn't be my approach to internet access for children, granted. My approach would be to keep the PC in the living room, where they can't use it without being observed. That assumes there's ALWAYS an adult with them, but there isn't any other sane way to raise children, IMO. In any case, if they're left unsupervised there is *NOTHING* you can do to prevent them from viewing random content on the internet (or, worse, on television), because they'll view it at a friend's house.