1. Kids this age are very concerned with their peers, stories dealing with group acceptance, overcoming image issues and supporting each other are good topics.
2. Middle graders know right from wrong and they feel very concerned with justice. Books where good overcomes evil are great, or bully's getting their just desserts.
3. At this age kids are getting into activities such as; sports, music, and drama. Using these subjects will help them identify with your characters.
4. As parents know, middle graders are learning to be independent and often challenge authority. Books with strong characters who hold their own are good subjects.
5. At this age kids are really into action and adventure, they want books that move and are enjoyable to sit down and read, without being forced to do so.
Inspiration from You Can Write Children's Books Workbook By Tracy E. Dils.
Image from http://yalitchat.ning.com/group/middlegradelair