How would you police that?

Well for starters you would not. Not the internet police anyway.

It is up to the parent to decide when their child is mature enough for mature content. If a police officer gets called to a situation they ask if the minor should be on the internet. Parent answers yes or no. If there is some manor of legal fallout as a result of them being online then it is just like we handle it today. If law enforcement get called they do their job and investigate what happened and gather details. Law enforcement have already arrested countless numbers of Reddit moderators, predators and all manor of other unstable people. The body-cam videos are great. If required they call in social services to interview the parents and children. This has already existed for some time. The internet does not require anything new. If the parents are facilitating a child using the internet when they are clearly not ready then the parents could be fined, have their children taken away, arrested, etc... depending on the circumstances and number of repeat incidents. The courts can handle family issues like they always have. If the child is being bullied the court can issue a restraining order. If a groomer is stalking the child the police can meet up with the groomer.

As for Discord they will likely be left alone just like 4chan as it is a great place to log what everyone is saying both text and even better, silently transcribed voice.

Are you clarifying the position of jumpcrisscross, or outlining an entirely different counter proposal here?

It seems significantly different in scope, intent, and impact.