I think traditional parenting is the control you're after - worked in our house so far. "Weirdos on the internet will send you disturbing images that give you nightmares, try and encourage you to do stupid things that will hurt you, try and get money of you, or damage our computer; I've set it so you can't make friends with them nor receive messages - if you want to add anyone, like school friends, that you know in real life that's fine, ask me first. If you mess with the settings I'll have to uninstall it (block it, whatever)."
