What not just have site that are serve explicit things advertise that. Have a large scale Blocklist, and if you serve explicit content, then you need to be on there.

Then have a option when people sign up for internet to block all the IPs associated with explicit stuff.

Adults and parents can choose "yes or no" when they pay for internet and then be done with it.

If children are still getting access, then their either technically savvy, which nothing will stop. Or an adult in their life is allowing unblocked internet.

Just make ip filtering easy and default so that parents can set it up without having to understand anything. It should be a required thing when you request internet access "do you want to filter out explicit content"

Some sites that have dual content may have to have separate IP addresses for non-explicit and explicit/non explicit. But that is relatively easy. And if they do not care, then they will by default end up on explicit list.