AIM uses port 5190 by default. Have a look at this to help you create filters on the fly (requires Win2K or better). Scheduled tasks may help you set time limits.
in reply to A perl personal firewall?
On Linux, you want iptables and cron. And there is a GNU AIM client, so if they know anything about Macs/Linux/not-Windows, you'll have to cover all bases. This is why the router in my house is built from a Linux machine (as opposed to that 'DSL router' crap). I like being able to set up timed, specific filters. BSD can do it, too; I just have more XP with Linux.
My Opinion: Let 'em use it after dark. Kick 'em out during daylight hours. That was (effectively) the rule my parents used, and I'm not sorry. :-)