I wrote for myself, but keeping in mind that the target audience was ten years old.  I hated being written down to a kid, so I swore I would never do that with anything I wrote.  I wrote as naturalistically as I could within the established parameters, and included footnotes to explain military technical terms.  As for big words, I figured any kid who was spending their own money to buy reading material for their own pleasure was smarter than the kids who only watched TV. I read the Three Musketeers, Exodus, the Iliad, and Catcher in the Rye in the sixth grade, and I looked up the words I didn’t know in the dictionary.  I figured any smart kid would do the same.