"My book is suitable for children of all ages"

In other words, you haven't a clue about the needs of specific groups of readers and you have no sense of what sort of book you've written. You need to get reading, get learning, get analysing. Become an expert. It shouldn't be too difficult because you are, are you not, a passionate reader of the sort of books you write? (If you aren't, notice my most crabbit expression looming over you.)