It's up to parents and culture and individuals. No state solution can meet the parameters of privacy and do anything impactful with regards to content filtering, and we should be fed up with "but think of the children" as a prelude to further losses of civil liberty.
And there really isn't any effective ways to stop them from finding this type of content. And I don't really trust any automated moderation methods either...
Only realistic option is curated content, but by whom and with what political agenda?
And what is the point, they can find it outside anyway, as there is no real way to lock down everything.