I have a question...

In a story of mine, I have a four year old girl whose knee is broken in a 3 am car crash. I think it's a hit-and-run deal, because everyone else at the scene needs to be unconscious or gone. Her father is in the car with her, driving obviously.

I would love to know how I can break her knee without killing her or her father, and preferably by throwing her clear somehow. (She needs to be semi-awake, because she needs to be found by someone. So what needs to be wrong with the car/how does the other car need to hit the car with my girl in it?