It was my job for a decade to try to keep tweens and teens safe online. Let me tell you what you already know: no law or technology can do it. There are no lengths kids won't go to to talk to friends without prying eyes. The harder you try to lock it down, the dodgier their solutions will be.
explain the risks and tech like you would to any adult, then step back, give them space, let them know they can come to you with anything (no blame), and let them try.
the harder you squeeze, the more things will squirt out in places you don't see or expect, and the messier it will be.
it's like abortion. you can't outlaw abortion. you can only outlaw medically safe abortions.
