I was reading a middle grade book last night, and what struck me about the author was the way she used very simple language to create surprise in her writing. Her style struck me because the last few middle grades I read used a lot of repetition to get ideas across.
I think repetition is a pitfall of any YA or MG writer, maybe especially for MG writers. We are never sure if kids are connecting with the ideas we are saying, so we, as adults, tend to hammer them in a little too strongly sometimes. We do it in life, so it's pretty natural that we do it in writing. Kids, of course, pick up on this instantly. Case in point:

The other day, Christopher, who is now nineteen, came with me to run a few errands. He works about 30 hours a week, maintains a good GPA, has a steady girlfriend, and just completed and passed two summer classes. So, really, I should know better. Dashing into the library, I looked over at the semi busy road and called to him, "Be careful crossing that street!"

He looked at his friend, grinned, and said, "Aw hell, Mom, I'm just going to shut my eyes, run into traffic and hope for the best."

I know. I pretty much deserved that.
So now that I am writing more full time, I am doing a lot more revision. (Writing to me really IS revision - I can't go forward until I've gone back and tinkered for a bit) My new editing eye is to delete anything that is not only obvious, but anything that is predictable.
This is harder than it sounds. I remember reading assigned novels in college and skipping over long passages of dialogue or description because I pretty much knew what it would say. That's the other piece: you want to make your characters consistent, but you also don't want to make them dull. And a lot of what I had to read in college were the kind of books where characters represented ideas or theories, so it may have been worse. In kid lit, the cliched characters are lethal.
All this being said, I will leave you with this bit of advice: