Whenever I see people say "oh we should just get the ISPs to block it!" I wonder if they work at any ISP, or are just an end-user consumer of their services... As someone who does work in the ISP/telecom infrastructure sector my first reaction is hell no. I do not want US/Canadian ISPs to go down the path of what has happened with, for instance, Spanish ISPs DNS-blocking football related things as a result of court orders.

I am not, indeed. However as programmer I was sometimes told to do what the customer wants, even though it is complex or inapt. If this DNS blocking is inapt or misused, it's a political problem so let the citizens deal with it (I know this seems like wishful thinking given the current poor state of politics at least in the EU, but hopefully we will fix that while it's still in our hands).

Besides, I read about what happened with Spain and football, and from my understanding the issue was that they blocked not DNS but IP addresses. And problem domain is also vastly different, a priori: in that case, they faced sites that were actively trying to avoid blocking, while in our case, I doubt pron sites and such will try to do the same, because children are not their target audience.

An alternative could be to create a top level domain (TLD) "for kids" and restrict kid phones to it. Then in your TLD you can admit sites that meet minimum criteria (no pron, no terrorism, content and chat moderation for SNS).