You're not a parent to any of these kids, you're genuinely concerned enough to put this much thought into it, but haven't told their parents a single thing about this, and you're asking a bunch of jerk-offs on the internet what to do ...? -_-'
The "responsible" thing, if you're actually worried about them, is to have a talk with their parents. If you're trying to be their buddy, of course, they'll probably despise you for a while. Maybe a long while.
The normal thing to do is, since you've already talked to them yourself, shut up and move on because--no offense--it's actually none of your business. :P If you're neither parent, nor legal guardian, it's not your right to censor the world or determine anything consequential about their lives.
The fact that you haven't told their parents and are considering forms of "punishment" on your own is unsettling. -.-' Furthermore, masturbation is never something to punish a kid for, you will screw their psyche up royally. Do I think porn is for kids? Nah. But if kids actively go looking for it, you will not be able to stop them, period. Educating them is a worthy goal, but again, it's neither your responsibility, nor--in all honesty--your right.