I always wonder how well a piece of realistic fiction will resonate with kids. This one is obviously targeted at a specific problem, the one of not being believed, and being misunderstood. The voice is uncomplicated, and uses some sophisticated vocabulary for the age group--which I think is great. We want kids to know that their thinking has real words. This is targeted at preschool, so it is most likely intended as a read-aloud. But it may be a good chapter book for K-1, too.
The writing did not seem condescending, which is what I look for as the common mistake when a writer is trying to inhabit the mind of a child.
I think this could be a good story for complicating bullying, because the main character does not realize that her own actions may be what is causing the little brother to want to blame Sadie & Ratz for everything that goes wrong. She is the bully, and while things do work out for the best, I don't know that the book is about her 'learning her lesson.'
This book makes me think about Bruno Bettelheim's work, and the idea that specific issues and conflicts may be better explored in fiction, especially with folk and fairy tale material--because we can exaggerate and amplify and even make characters out of the issue. I'm often suspicious of issues-based realistic fiction for this reason. For this one, because the writing did not feel didactic or condescending I felt okay giving it a 3.