We know what good UX is already from history. Movies for example have an age rating system that clearly lists the ages the movie is suitable for. If you look underage, you get asked to provide ID.
Parents do not have the time or energy to configure modern parental controls which work more like tuning firewall rules.
The other part of the problem is you need collective action, if you ban your child from Facebook, they won’t get invited to parties anymore. If all children get banned, the parties will be arranged over sms or some other non social media. As it is today, banning your kids from social media is overly punishing since all the other kids are using it.
That was my point though.. showing ID at the door doesn’t translate well to the Social Media landscape.
If we actually cared about supporting children and parents as a country, parental controls and access, and making sure that systems were actually safe and protected privacy, would NOT be an afterthought.