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, they regard & defend parental responsibility & discretion in leaving access open to children, and they find measures "enterprising and disobedient" children can circumvent preferable over unacceptable alternatives.

The Commonwealth argues that central blocking would not fulfill the state’s compelling interest as effectively as the access number does because minors with phone lines could request unblocking or could gain access to unblocked phones. It also argues that a parent who chooses to unblock the home’s phone to gain access to sexually explicit material for himself or herself thereby places dial-a-porn phone service within the reach of minors with access to that phone. In 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. See Bolger v. Youngs Drug Products Corp., 463 U.S. 60, 73-74, 103 S.Ct. 2875, 2883-84, 77 L.Ed.2d 469 (1983) (parental discretion controlling access to unsolicited contraceptive advertisements in the home is the preferred method of dealing with such material).

Even with parental control, the Commonwealth is undoubtedly correct that there will be some minors who will find access to unblocked phones if they are determined to do so. As the Supreme Court noted in Sable, “[i]t may well be that there is no fail-safe method of guaranteeing that never will a minor be able to access the dial-a-porn system.” 109 S.Ct. at 2838. Nonetheless, the Court did not deem the desire to prevent “a few of the most enterprising and disobedient young people,” id., from securing access to such messages to be adequate justification for a statutory provision that had “the invalid effect of limiting the content of adult telephone conversations to that which is suitable for children.” Id. at 2839. We hold that because the means used, requirement of an access code, substantially burdens the First Amendment right of adults to access to protected materials and is not the least restrictive alternative to achieve the compelling end sought, the statute cannot survive the constitutional attack.

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.