The US federal courts had an interesting opinion there: parents may always allow their children to access protected speech. Even with sex-related materials, the Supreme Court has stated

the prohibition against sales to minors does not bar parents who so desire from purchasing the magazines for their children.

They regarded as constitutionally defective laws that impose a single standard of public morality. Instead, they’d allow laws that “support the right of parents to deal with the morals of their children as they see fit”. Laws that take away parental control are also impermissible.

“It is cardinal with us that the custody, care and nurture of the child reside first in the parents, whose primary function and freedom include preparation for obligations the state can neither supply nor hinder.” Prince v. Massachusetts, supra, at 166.

In [another decision][shelf], they regard & defend parental responsibility & discretion in leaving access open to children.

The Fabulous Associates […]. Id. at 788. The court noted that “[i]n this respect, the decision a parent must make is comparable to whether to keep sexually explicit books on the shelf or subscribe to adult magazines. No constitutional principle is implicated. The responsibility for making such choices is where our society has traditionally placed it — on the shoulders of the parent.”

So, according to them, presenting such content to children ought to be left up to their parents, and laws shouldn’t infringe on their right to do that.