To my grandparents / great grandparents generation at the time (70/80's) TV has become the world's nanny would have generated all the same comments as this.
If you watched tv - the idiotbox - for more than 15 minutes your eyes would go square.
That is simply not true, with television you also had the panic of "brain rot" but at least you could still trust public television networks with not showing dubious content.
Youtube for kids seems to be a rabbithole place where you end up with "Spiderman kisses Anna", very obvious advertisements or very trippy shows while on public television you had kid shows with educative touch on them (a visit to the forest, children songs,...)
I allow my children to watch a very curated list of things and free reign on the Belgian "kids channel" but Youtube? Never.
Cartoon Network for example has always had advertising aimed at children, and some shows that definitely was considered dubious in my household growing up (in far away Romania), such as Cow and Chicken and that show's obsession with butts. And there was generally no educative touch to these shows, they were mostly interested in children's entertainment. The same was true for most cartoon shows I saw growing up - they usually didn't contain explicitly educational content, though of course they were incomparable in quality to things like the spam on YT Kids.
See the critical reception section for highlights including; Sesame Street also introduced children to a shallow pop culture, undermined American education, and relieved parents of their responsibility of teaching their children how to read.
"That is simply not true, with television you also had the panic of "brain rot" but at least you could still trust public television networks with not showing dubious content."
No you couldn't. TV has always been used for nefarious purposes, especially political.
The other difference is that with public TV there will be moments where it shows stuff the kids couldn't care less about, and they'll go do something else. With limitless online streaming, there is always something to keep their attention.
There were plenty of cartoon-only and kids-only TV channels when I was growing up. Of course, I sometimes got bored of the shows, but kids also sometimes get bored of YT.
For it to be the case I would expect to see a split in the population on many metrics, especially around education, dividing the with / without TV / YouTube groups.
Presumably the adverse effects would overcome the dominant factor which I assume is socioeconomic.
As for the making your eyes go square - I doubt youtube has any more effect that the old tube tv! Its also believed that reading books gives you bad eyesight ...
If you watched tv - the idiotbox - for more than 15 minutes your eyes would go square.