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.
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.
(Also, I find it weird to give a kindergartener unsupervised access to any internet device. They don't understand the threats they're confronting.)
That was the selling point of Youtube Kids—it wasn't supposed to be unsupervised access to the internet. It was supposed to be curated.