I wonder whether the book entitled "for Kids" can be understood by pupils aged 6 or so. The Japanese word corresponding to the English "kids" may suggest that it's for the pupil whose age is at most 10. So teens in Japan would feel the book "for Kids" is too stupid to read.

It's not because Japanese teens more strongly want to be regarded as grown-ups. In Japan, most children in elementary school can read books because it's fairly common to let toddler learn to read at home. The point is, it doesn't mean all of them can actually understand the content they are reading. Indeed, there's a lot of books that might not be understood by most teens, even if the words and the expressions in them are easy enough for mere kids.

Apparently, the skill of reading books scarcely has to do with the ability of understanding abstract concepts from books. On the other hand, as I said above, most children in Japan can read books. That's why the book "for Kids" is miserably avoided by teens. Furthermore, it would be criticised as not being appropriate "for Kids", if it's filled with paragraphs, even the funniest and easiest ones.

It may sound peculiar, but the fact that most adults in Japan have never read any books except cartoons explain the situation too. If you'd like to sell books "for Kids" in Japan, they must be a form of cartoon, or messy chunk of visual elements without long paragraphs. The book with pages of relatively easy and funny texts would never be considered "for Kids", despite all the kids can literally read it.