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.
(Also, I find it weird to give a kindergartener unsupervised access to any internet device. They don't understand the threats they're confronting.)