the entire site is denied to kids

I also covered that here [1]. Indeed if parents do not enable all of Youtube or Youtube does not move most adult content into a unique URL or their server does not send the header for anything flagged as adult the kids will not be advertised to. They would have to go to a kid friendly site that moderates before a video is viewable or Youtube would have to change moderation tactics. Kids need not visit Youtube. There are kid friendly sites.

[1] - https://news.ycombinator.com/item?id=46152727