Let's talk about children's lit. You've got picture books, chapter books (aka middle grade) and young adult. People usually separate them by age, but it hit me that on an individual level, they may be more about how a kid feels about the opposite sex (or whichever sex they are attracted to.)
Chapter books are for when girls and boys are friends.
Middle grade is for when girls and boys don't like each other.
Young adult is for when they do.