When I wrote my debut novel, The Hundred, I wasn’t thinking of children in a specific age range who might read and enjoy the book. I just wanted to tell a story. When it was finally edited and complete at a whomping 500+ pages, I realized that I had a dilemma on my hands.

People (and agents, in particular) think that long books for kids are a death knell. Everyone advises against them, unless you are big A+ author whose editors have been cowed into submission and won’t delete nary a word. So m...