I work for a kids entertainment company, so I've totally been researching this stuff. Here's a rundown.
Basically, Youtube creators are mostly doing nothing wrong (but some of them are). Youtube is doing a lot wrong, and they're trying to put the responsibility onto the creators.
So basically... COPPA means that companies shouldn't collect data on kids, like their name, address etc. There are also rules that kids shouldn't be able to private message or post comments or otherwise given opportunities to share details like their real name, address or phone number. If kids DO share this information, even if they do it by blatantly flaunting every rule and say, drawing a picture in an inbuilt oekaki system and carefully concealing their phone number in the picture (you may laugh, but this is a thing that happens on Popjam, for reals. Anyone who has or works with kids knows that kids are not always sweet little darlings, they can be manipulative and cunning little s), it's on the company, not on the kid that did it.
So... where does Youtube come in? Okay, so... to have a "grown up" (standard) Youtube account, you need an email, and youtube needs to collect the data of your email address. For that to be legal, the person must typically be 13+ years old (though this actually varies by country), which is seen as old enough to have the understanding to agree to give away that data and what it means to do so (kinda like getting your first proper bank account or doing a part time job, also things in many countries you can do at 13).
So for younger kids, Youtube made... Youtube Kids! ...Which kids over age like six don't want to go on... because it's called "Youtube Kids" (let's face it, if you were eight, you would probably see yourself as being grown up enough for proper youtube and watching the kind of thing your older siblings watch, just like when I was eight I wanted to listen to Alanis Morisette like my cool teenage big sister, not like... nursary rhymes or whatever).
So since the actual children who aren't pre-school age are on standard Youtube, companies like Lego and Cartoon Network have set themselves up on standard Youtube where the audience is... and here comes the problem.
So, Youtube has a bunch of people on their platform making content that is blatantly meant for kids, because everyone knows kids are using it, even though they're not supposed to be using it. Is that bad? Well, for Youtube... yes, yes it is. COPPA violations come at about $40,000 a pop, and usually if a person is violating COPPA, it's not just once. Companies have already incurred millions of dollars in fines just for things like not paying enough attention to what data their third party analytics software has been collecting.
If somebody can prove that Youtube is knowingly hosting content intended for kids under 13 on their platform, and has not made every possible effort to stop this happening, the amount that Youtube could be sued is... potentially astronomical.
So their first action was to say to the bigger creators like Lego, "Okay no no no, if you're posting videos on normal youtube, they can't be for kids! We'll have to kick you off the platform if you do that!" to which those creators responded, "Ohhh haha noooo we're not making this for kids!", and so you get things like the Lego channel never addressing the audience like "hey kids!", never featuring any children in videos, never speaking about lego as a children's toy except in a context like "this would be a great present for your child, adult viewer!" ...basically, all the channels making kids content pretend that they genuinely believe that they, the Lego/Cartoon Network/Roblox channel are making content intended entirely for an adult audience.
But now it seems like that isn't enough any more, because it's so blatantly transparent. So Youtube now wants creators to say on their videos "THIS IS NOT FOR CHILDREN, IF YOU ARE A CHILD, STOP WATCHING THIS!" to put the onus onto the people making content because of their complete lack of ability to stop kids from being attracted to their platform, making illegal accounts and ignoring the alternative provided because it's just so uncool to older children.
If you don't do it, you shouldn't be the one to incur a COPPA violation, but it's possible Youtube could turf you off their platform for encouraging kids to their platform so that Youtube might incur one.