What content shouldn't children be accessing? Is the content a 7, 11 or 16 year old shouldn't access different between age brackets? Who makes that determination? Is this access restriction at the whole site level, or per-post? Does safe-harbor apply, or is a site-operator liable for age-inappropriate content it hosts for its clients? On S3, for example, is each object tagged with an age category, or would it have to be a totally separate S3, like GovCloud?
It requires manual moderation. Companies like Facebook, Google, and co. have spent much effort telling you that's impossible. In fact, that is a type of half-truth that is a lie. The full truth is that it's impossible _at their scale_.
Their business models require little to no human moderation, because it simply doesn't scale (for their business to stay profitable).
Personally, my feeling is that if you can't take care of your product, you should go out of business.