YouTube Kids does have a way of turning on curated content only[0]. The main problem is that when you do that the library is tiny (and seemingly stuck in time).

This isn't the default though: Parents have to actively enable it. But it does get rid of most of the toy/McDonald's/etc adverts and "child personalities" that are prominent on the platform.

[0] https://blog.youtube/news-and-events/introducing-new-choices...