I think that's over thinking it a bit, youtube algos are easy to manipulate and a single thread on 4chan can create a fair bit of weird media, for the explicit intent of creating weird media.
I do prefer the 4chan theory, since it satisfies Occam's Razor. That said, the frequency of new videos from multiple channels dedicated to nothing but these videos, as well as the monetisation, makes it unlikely. Also considering the number of subscribers the subreddit has I'm sure that if there were any new 4chan threads suggesting people make those videos it would've been spotted.
It's honestly the sheer number of videos that is the shocking part of the mystery. It's not just a few dozen videos from different animators, it's hundreds of videos from dozens of channels pumping this stuff out. Unfortunately I find it unlikely that we'll ever find out the cause.
The cause is algorithms run amok. Find popular keywords: Elsa, Frozen, Fidget Spinner, Sexual. Have an AI mix these in whatever weird way and churn out nightmare fuel to kids, in exchange for big fat ad revenue. Kids, more easily manipulated, and with a whole life as consumers ahead of them, are valuable targets...
Both can be true at the same time. 4chan is basically about pointing and laughing at things on the internet, right? So it doesn't matter much whether they're gaming YT's algorithm to show disturbing things to kids, or pointing to other people doing it.
(Also, I find it weird to give a kindergartener unsupervised access to any internet device. They don't understand the threats they're confronting.)
Letting a multi-billion-dollar surveillance company show more or less whatever it wants to very young children seems like a bad idea. We tried a very primitive version of this model a couple decades ago, with Saturday morning cartoons interspersed with ads selling sugary cereal, and the results were not great.
It's honestly the sheer number of videos that is the shocking part of the mystery. It's not just a few dozen videos from different animators, it's hundreds of videos from dozens of channels pumping this stuff out. Unfortunately I find it unlikely that we'll ever find out the cause.