This differs in no way, except for the name "YouTube", from the objection to children watching television that goes back several decades.
Most children's television content has some value: a lesson, moral or at least an actual story.
Others have mentioned the horrifying inappropriate content, but even ignoring that, there is a mind-boggling amount of content on YouTube that is really devoid of value (at least in terms of what I want my kids watching):
* Nursery rhymes -- which is not so bad, but it just keeps going forever
* People (often adults) dressed up as children's story characters, just goofing around
* People (not always kids) literally just playing with toys
* People (not always kids) doing some mind-numbing repetitive task (for 10's of minutes) like opening kinder eggs, molding play doh or pouring glitter on things
For instance, I suspect a lot of kids do not have proper fear of wild animals, because from television they know animals are friendly, and some can even talk.
Additionally, bad guys wear black and have scars, while good guys are clean shaven and wear attractive clothing.
There was a whole “panic” at my kids school about not warning your kids not to watch “Momo” videos.
Unfortunately, these are now unbelievably vapid and pointless, even on PBS. It’s not quite clear what happened. The comparison to early Sesame Street, Mr. Rogers’ Neighborhood, Newton’s Apple, Long Ago and Far Away, Reading Rainbow, Bill Nye the Science Guy, etc. is stark. I’m not convinced most of the current shows are any better than watching an endless stream of poorly produced nursery rhymes on youtube.
I wouldn’t let young children spend nontrivial amounts of time watching “Kids’ TV” in 2019.
There's always been problematic or otherwise weird children's content, and content that was mostly just an ad (80s cartoons were bad about that), but some of the stuff you can find on YouTube is in a whole new class of questionable. At least some of those 80s toy commercial cartoons were still entertaining in some way.
In 10 years people will be looking back at this in shock that it happened and there will be downstream affects on the kids exposed to it. It's literally programming weird stuff into the psyche of these young children.
~20 years ago people were deeply concerned about the impact and influence that sophisticated television and radio advertising campaigns were having on people, culture, and the economy.
It's gotten so bad now that we seem to have forgotten about how creeped out people were about the comparatively tame advertising through broadcast media. It's not safe to assume the world will march forward to be better or more moral as time goes on. We're just as likely to regress as progress, and the myth that progress is a passive endeavor makes it more likely that we won't.