The way I see it, parents shouldn't let their children use the internet unmonitored if they are worried about them seeing adult language. Let's face it: if you're going to use the internet at all, you're going to see swearing at least once in a while. There isn't really anything you can do to avoid it. Even if mature content is forbidden on-site, there isn't anything that can be done to control online interactions (hence why the rating systems never cover online multiplayer on games).