Adults often buy YA ebooks to read themselves.
Comparatively, teens rarely buy ebooks to read themselves, and I would bet that the 12-and-under crowd buys ebooks even less than rarely. :-)
Then we loop back to picture books, which I think adults sometimes buy as ebooks to entertain their kids on screens on long car trips, etc.
*Yes, all my knowledge here is anecdotal based on my work as a YA Librarian and self-pubbed YA author, FWIW. ;-)
All that to say, I think 99.9% of all of us would have a very hard time selling ebook copies of a chapter book or MG novel.