I agree that there is - of course - a parental obligation. Why isn't it statutory? We have laws about car seats at one end of the spectrum, and no-one thinks that's a step too far (now). At the other end of the spectrum, I bet every single one of us would agree that social services should get involved if a parent exposed their child to other people having sex (which is against the law). Why not a statutory obligation to take reasonable measures - blockers at device-level and household level?
There's also no reason why phone companies shouldn't shoulder the responsibility for establishing new phones (without device-level blockers) are going to adults only, like supermarkets selling alcohol or cigarettes. Of course there will be shit parents who lie, but many many more will not.
What's required is a system of measures that change the culture and make it harder (though not impossible - of course) for children to slip through the net. But this is a good first step.
The trouble is that third-wave feminists have been persuaded by men that porn is empowering/a right, and those men themselves don't give a fuck about women or children, as evidenced by the fact that they consistently push away the evidence of widespread trafficking, lack of meaningful consent, and child abuse. PornHub removed something like 80% of their content because of possible child abuse. Eighty per cent. Men know this, but they don't give a fuck. There are endless excuses, but the bottom line is that if men gave a fuck whether their porn was consensual or not, whether a girl was under-age or not, the front page of PornHub wouldn't look how it looks. They don't care.
So there's no outcry for governments to respond to, and in any case the governments themselves are broadly constituted of men/intersectional feminists. It amazes me that we can look each other in the eye to be honest - it's astonishing what we've allowed to be done to our children.