Then the parent(s) need to be parent(s) by monitoring their childrens online activity if they feel theyll have access to content they shouldnt. They should also explain what porn is, why they shouldnt view it, and what about it is harmful to prepare them for when they will, not might, but will encounter it so they have the tools and understanding necessary to handle and process what they are seeing. Porn exist. Its not going anywhere anytime soon. Making it harder to access for everyone isnt going to make it go away, stop witty teens from finding it, or stop content that slips through the moderation cracks in spaces that dont allow pornagraphic content.

Im not advocating for more porn or easier access to porn, but rather recognition that the parent(s) are responsible for their childrens wellbeing, education, and preparedness for adulthood where they will absolutely encounter adult content, online and off, regardless of if they voluntarily sought it out. Hiding it, pretending it doesnt exist, or avoiding the topic with them doesn't prepare them for reality in adulthood where they are expected to be able to handle uncensored life.

Not to mention, as it was pointed out several times in the comments, an ID mandate for a website is extremely easy to circumvent with a vpn, something that is incredible easy to obtain and set up. The only way, in my opinion at least, to effectively stop children from accessing porn is for the parent(s) to monitor their online activity and educate them on what and why this content is not okay for them to view as a child.