Ya know, you kinda make me want to write one now!
I really don't know a specific book for that age. We had a discussion of general children's book, but he is too old for these I think ...
https://www.treeleaf.org/forums/show...recommendation
Is he sitting on his own? In our house, my wife and I decided not to compel our kids to sit when they were younger, believing that they would see and understand the general reasons, and might decide to do so when grown. (My son famously once asked if "daddy is bad, because he is always sitting in the corner!?"
). We believed that it is best to create a house with Buddhist values, and then they would get the message that way.
Soto-shu has some online Manga, but they are a bit religiousy (the Dogen legend, for example), not particularly about "how to Zazen" ...
https://www.sotozen.com/eng/library/comics/index.html
Let me ask at the Soto Zen Buddhist Association if any of the Soto teachers there know of a book.
Maybe the best if for you to explain to him.
Gassho, Jundo Dad
STLah