I believe the only correct way to solve this is to add an RTA header on sites that permit user-generated content and require the most common devices and browsers to once again recognize this header and instead of just treating it as adult content, also use it for user-generated content. Then put the liability on the parents to lock down devices. Let parents decide which domains/urls to approve-list in the devices.

If a site with user-generated content is missing this header give them a warning, then fine them {n} percentage of their net revenue per day until remediated. After {x} days out of compliance seize their domains.

If a child/parent is having compliance issues on their devices leading to problems, hand it over to social services like they would for any other social/family issue. Social service issues are not tech platform issues.

Using RTA in HTML:

    <meta name="RATING" content="RTA-5042-1996-1400-1577-RTA" />

PHP:

    header("Rating: RTA-5042-1996-1400-1577-RTA");

Apache:

    Header set Rating "RTA-5042-1996-1400-1577-RTA"

HAProxy:

    http-response set-header rating "RTA-5042-1996-1400-1577-RTA"

NGinx:

    location / { add_header Rating 'RTA-5042-1996-1400-1577-RTA'; }