My ten year old found it compelling, but depressing. Without an understanding of the hideous weapons used during World War I, the descriptions of the injuries were difficult to understand. (Not that I'd have wanted them to be graphic.)
A child might take the moral of this scary fairy tale to be "Be very careful what you wish. Very careful." And, yes, I approve of that message as it's presented in Into the Woods. "Be careful the wish you make. Wishes come true. Not free."
And I do think it's good for all people to think through the full ramifications of their actions and their wishes. And through the main character, the book does help us to think about how to formulate effective, clear, less dangerous wishes.
But do we want our children to worry that their possibly less-than-perfect wishes will come true?