A pointer about age – children like to read about other children either their own age or a bit older. They tend not to be so keen on reading about younger children – not as the main characters, anyway – they have already been there and done that! So if you want to write for 8-11s, make your protagonist 11. If you are writing for 12-14s, make your protagonist 14 or above.
Oh, and girls will read about boys, but boys aren’t generally so keen on reading about girls. Just saying!