This is for a home / family network?
Has Facebook turned on SSL by default yet? I know that Twitter has, and Facebook has the option, not sure if they've thrown it on by default yet?
In any case, if they haven't, I imagine that it is coming, and then sniffing out contents of messages will not be so simple. You'd have to install a man-in-the-middle service with a fake SSL certificate and install said fake certificate as trusted on all of the client machines. (Good luck doing that on the iPhone.) And that's just to be able to see them in clear text. If you're trying to scrape them out, you're going to be constantly fighting with Facebook every time they change up their interface. Are you going to be tasked with updating this every time a new social service or game comes along?
It seems like the better approach may be to just have them learn some basic Internet safety.