Parental controls only need look for an RTA header [1] that would need to be legislated to be served from any adult or potentially adult user-generated content site. Not perfect, nothing is but it would take an intern maybe half a day to add the code to clients to check for said header. Adding the header on the server side is at trivial. Teens will bypass it as they can stream and watch together porn and pirated movies in rate-PG video games that allow defining a "movie player" but small children on locked down tablets would be fine.

[1] - https://www.rtalabel.org/index.php?content=howtofaq#single