The .kids domain has been suggested and I think someone even pitched the idea of a TLD but someone decided it wasn't "viable"

As for tags... mandatory tags could definitely work. It's a simple matter to add a rewrite command to Apache to insert the tag on any page on an adult-related domain, so it wouldn't have to be inserted on individual pages.

Outside the US it might be a bit more of a challenge to get compliance, but if the rule said that any site viewed in the US had to have the tag, a noncompliant site could either be blocked by IP address or the originating ISP could be instructed to block outbound traffic to the US until the tags were in place.

I agree that it's ultimately parents responsibility, but I think that the industry can take steps to help that won't significantly impact our bottom line.