"1) WTF are 12-yo doing unsupervised on the web with apparently no clue whatsoever? Surely they don't have their own connection, nor can they afford computers or smartphones by their own means whithout the parents knowing?"
Are you actually expecting every parent to monitor every device that a child get close to. phones / computers. etc. I think you need to live in the real world.
The easiest way would be by phone, most kids that have one now have some form of top up system that still lets them have facebook access for example. So uploading and sending pictures and video is no easy. there is no way it can be controlled unless they don't have the devices, but there will always be a friend that has one.