> Is the child going to inform on themselves? No. Is the adult, when they don't even know about it?

In the context of social media, if they want to actively participate they have to given that it's the entire point. It's true that even with a government ID scheme people could borrow someone's ID to get passive access with their consent. But a kid couldn't share an account with a parent without that parent knowing because you see their activity, and they also couldn't post.