In my experience, one of the biggest mistakes made by new authors is not understanding the difference between writing for children and writing for young adults. I started reading adult books when I was ten years old, primarily because I found teenage literature to be condescending. Ok, that was a long while ago and young adult fiction has come a long way since then, but not all new authors seem to have noticed.
J K Rowling’s Harry Potter series demonstrated that good children’s literature can have appeal across the generations, but it is worrying how many new authors write for a genre that they haven’t read since they were young.
The strongest advice I can offer is to go to your local library and ask them which are the most frequently borrowed books for the age group that you want to target. I am not suggesting that you copy these books, or their style, but the research will inform your own writing, because you will have a greater awareness of what appeals to your target audience.
Writers tend to fall into two camps, those who have a life work that they have to express, regardless of its prospects and those who want to succeed at writing. Most successful writers are those who write to sell, a happy author is one who can make their work commercially acceptable without having to compromise too much of their writing soul.