Earlier quoted context omitted.
Kids don't need Twitter. There would obviously arise a market for a moderated, or more regulated, version of this type of social media. Social media doesn't deserve an exemption from the efforts society makes to protect children in other domains.
So basically you just ban any company from serving both kids and porn stars, no matter how hard they keep them separated? This is one of the avenues people often go down to censor the porn industry. You're effectively doing "think of the children! ban porn!" without doing it explicitly.
Those that don't, or don't care, would not seek a rating, and would not be accessible to devices that parents control. Otherwise they would be free to host whatever legal content they wish.