OK, that's just not true. There's a ton of great stuff on YT Kids (though as I mentioned not enough). But yes, you have to find it for them and show it to them yourself. Left to their own devices most kids aren't going to make good choices. They're kids, that's the point.
This is just the "be a parent" thing, really. There's absolutely no way to get around the need to educate your own children. Certainly Google isn't going to do it for you. And Google certainly didn't invent junky content; I remember staring at Hanna-Barbera garbage constantly as a child.
The funny thing is, I had to choose my child's age range (or enter the birth year or something) when setting up the app. Then, when I went to whitelist channel by channel, it had a bunch of recommended channels full of good/decent content. That should have been the default whitelist out of the box.
It isn't just about "being a parent". My daughter is very smart for her age. My wife used to be a teacher, but since we had kids, she stays at home with them. She has taught my kids a lot. She has learned a good amount from content on YouTube, Netflix, and Disney. I think it's good that she can use an iPad better than my parents (even though she can't read). She also can solve puzzle games on her own. Of course we guide her and limit her usage. My point is that an app marketed to kids should not be full of garbage content.
You have to whitelist a few channels, then it works ok. Though my daughter is only 3 now, and would be happy watching Blippi all day. Seems like it would be more complicated to whitelist channels as kids get older.