This is not a light book at all. And while the age of the heroine would indicate it at a mid-grade novel, I would not recommend it for young children. At least not ones who can't handle the very mature themes of the novel. This is a book about genocide, racism, hatred, and overcoming all of that. But in order to overcome them, the book takes you through some very dark paths. This means of course, that the ending is an earned ending and doesn't feel manipulative in any way.
Also, I'm in awe of Hardinge's ability to plot many threads and then draw every single one of them together for her conclusions.