in reply to
A perl personal firewall?
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.
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. :-)