All right, we've all heard this one before. But with children's lit it takes on a new perspective. Reach back into your childhood. Try to remember how it felt to be a kid at five years old, at eleven, and again as a teenager. Pick out specific times and instances that stand the most clear in your memory, and try to put those same emotions into your characters.
Kids have to solve their own problems.
Just as an adult protagonist has to overcome, or be defeated by, their own actions, so does the hero of your middle-grade adventure story. You can still have adult characters in the piece, but your protagonist needs to win or lose on her own merit. Kids don't like being hit over the head with the point of your story. Neither do editors. If you do have a point to make, be certain it fits the story, helps resolve the crises, and is woven into the character's words and deeds.