Squidguard, as I recall, blocks things by domain, or by url. You should be able to block something like youtube.com/adult in the URL list file. Just a domain name, something like youtube.com, would be blocked in the DOMAIN list file.
Check here:
http://www.squidguard.org/Doc/examples.html
or here:
https://help.ubuntu.com/community/SquidGuard
The Ubuntu documentation is easier to follow, and a little more complete, IMO.
One thing to be aware of, if you block advertisements, it will also block the .css files of youtube, making the site look ugly. Just a warning, if you end up doing it.
On another note, I tried squidguard, and it was good, but I felt that dansguardian was easier to set up and use. Squidguard can be difficult in some ways because you have to compile the list files into a format it can read. With dansguardian, you can use plain text files. I recommend dansguardian myself. Just a thought.
