That's exactly what the FTC fuss was about, though. It's easy to say that until a parent gives their kid access to it and suddenly YouTube is advertising and recommending stuff to a child, which the law doesn't provide a safe harbor from (not for the content uploaders, either, which is going to cause a lot of problems). The FTC guidance was to identify kids instead by the content they were accessing. Hence Google's complaint: https://youtube-creators.googleblog.com/2019/12/our-comment-... (tl;dr skip to the "Treat adults as adults" section)
edit: now I'm confused because your comment below (https://news.ycombinator.com/item?id=21975870) indicates you already know your idea here isn't going to work for the FTC?
To be blunt, even the FTC's own proposal isn't going to work for the FTC (at least not to the degree they want) -- it's a bad solution with unintended consequences. I've brought up a few problems, but other people have brought up far more of them. At a certain point, beyond explaining the FTC's policy and showing the problems, it's not really worth spending the time trying to come up with a way to make it work. It's better to come up with solutions that would address the concerns of actual parents, like jedberg.
Importantly, the FTC's approach is the opposite of what parents like jedberg need. Parents need a highly curated feed that they can feel safe showing to their kids. The FTC's approach is going to lead to creators erring on the side of labeling too much of their content as kid friendly. It's also going to force Youtube to get more aggressive with its algorithms, which will inaccurate label objectionable content as kid-friendly.
Parents needs fewer videos more carefully classified as kid-friendly, and the FTC is making sure that a lot more videos will be carelessly classified as kid-friendly.
Like I said below, my assumption based on the FTC comments I've read/watched is that they just haven't thought that far ahead. They're mad about Youtube in general, and they see creators as an easy way to get at Youtube. That's maybe an overly-cynical take on it, but I'm not super-inclined to be charitable to them on this one.
It's fine for Youtube Kids to be a much smaller selection of manually curated videos from specific partners, because Youtube Kids isn't replacing Youtube as a whole.
Given that their current solution basically kills ad revenue on those videos anyway, it's not like creators are going to be clamoring to be included on Youtube Kids in the first place. There's no harm in having a subset of Youtube that is highly gatekept, the same way that subsets like Youtube Red were.
Go ahead and let Youtube itself stay as a kid-unfriendly, algorithmically moderated wild west with hidden bombs of objectionable content spread around. That doesn't need to be gotten rid of, just have one corner on a separate subdomain that's nice and safe for kids.