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).