I agree with lupine. Get some kids to read it. Kids are generally quite picky about what they read, what with having shorter attention spans etc, so the simplest, most surefire way, to know is to ask. That said, kids also in a sense are more open minded, because they bother a bit less about writing style, and more about the storyline. But don't get complacent, as they do know when something is compete rubbish. Just keep in mind that what one will like, another might not, and vice versa. So ask both girls and boys, and different kids who prefer different genres, and the chance is one of them will like it.