That's probably a weakness in the algorithm. YouTube has never really been safe for children to just go about browsing. You need some serious curation if you want to actually display only child-friendly content that you can confirm isn't subversive or randomly spliced with crazy shit, or potentially some trippy as fuck algorithm nonsense.

If the algorithm is looking for videos with the same words, or associated popularity, or even just similar colors and shapes, it's not really a good algorithm for children.