Characters must be genuine and likable. The characters are what drive the story. The reader must love your main character from the beginning on. Give them characteristics that set them apart from others. What makes them who they are? What are their interests, dislikes, passions, weaknesses, and strengths. I love the Gilda Joyce series, by Jennifer Allison. Gilda is quirky, funny, and real. Sure she thinks she has psychic powers and finds herself in ridiculous situations, but we as readers want to feel that she is psychic because we like her.
Use humor. This I think is one of the greatest tools for middle-grade authors. Readers want to be entertained. Think about what makes kids laugh. Observe them. If you see some kids laughing about someone farting in public, you better believe they’ll laugh about it in a book. I love the book Charlie Joe Jackson's Guide to Extra Credit, by Tommy Greenwald. He knows how to add humor to the story. Kids laugh at the most bizarre things, and Greenwald shows that.
Give it conflict. Kids want to know that they can handle situations just like adults. They have the need to be in control. Let them see how the main character reacts when the stakes are raised and they are about to lose control. Make sure to set the stakes high. In the Harry Potter series, by J.K. Rowling, the risks and conflict only grew worse and worse as the story grew. The struggles that Harry goes through are what make us as readers love him and want him to succeed.