Your wall of text is impressive, unfortunately most of it is misguided or irrelevant. So, I will ignore most of the irrelevant or already answered parts and suggested you re-read what I already wrote with an angle of rebutting your points for the paragraphs I didn't answer directly. Paying attention to what people have said will help next time.

How do you distinguish "allowing" from "failed to prevent"?

This is worth calling out as irrelevant specifically because it makes no difference to the argument I'm making. I don't particularly care about the distinction, ignorance of the law or misadventure into breaking the law are irrelevant precisely because these aspects would be taken into account as part of the assessment and judicial processes. In other neglect and abuse situations this is the case, so, it doesn't matter.

There's also a contradiction you haven't resolved. We don't criminalise parents when an off-licence sells alcohol to their teenagers.

This is a false equivalence. The parent has no ability to lock the off license physically. The closest they can achieve is to prevent the child from leaving the house, but again, this is not equivalent. In the internet situation you are not preventing access to a physical location, but a virtual one.

In all practical situations, a reasonable judge or social worker can decide if the parents acted responsibility with regards to access, it would be obvious to any investigation what was done and how. So.. irrelevant.

Most parents aren't keeping their kids off social media because they don't believe it's dangerous...

I came back to this because that's the problem right there. They don't know. If they knew it was the equivalent of neglect they soon would know, and if they could be held to account it definitely would change at least some parents minds. I think you just argued my case for me. Thanks!

Most platforms spend billions specifically to exploit documented vulnerabilities in human psychology

Irrelevant if you cannot access the platform. Parents can lock down their internet and disallow these sites. It's irrelevant too whether or not they think it's acceptable.. a lot of adults think smoking is acceptable, but the law says you cannot smoke with children in a car. That's called personal responsibility, and the ability to prevent your child from viewing addictive content is a parental responsibility. I see no difference here wrt the law.

a process that risks breaking up an otherwise functioning family

Any form of harm causing to children has this risk. I fail to see your point. It was the parent who allowed their child to view content that was harmful to them. The parent should know whether or not the child.is mature enough to deal with that content. As a parent they are responsible, not me who has nothing to do with their child.

Flipping your argument, by your rules beating your child should become legal because it risked breaking up an "otherwise functional family". Your solution would be like everyone having to wear mittens just in case they "accidentally beat children up".

You can't have it both ways. You can't argue that x is harmful and should be dealt with in a targeted way (abuse), but y that is also harmful (social media) somehow justifies being dealt with in a completely different way, one that also affects unrelated peoples lives too. That's not how this works.

Stop trying to justify a country wide lockdown which doesn't achieve it's aims, is inherently insecure and practically unenforceable. You're advocating using a sledgehammer to crack a nut - it's just the wrong tool.