I think it would depend greatly on the maturity of the ten year old. And yes, it is partly about rape. The protagonist's father is defending a black man who has been accused of raping a white woman. You can find a plot summary here
http://en.wikipedia.org/wiki/To_kill_a_mockingbirdIt might even be a good one for the two of you to read together. I see no reason why, at 10 years old, you could read it to her and discuss the themes together. It is more than just the court case but what it is to be a tolerant person, father-child relationships etc.
I love this book and would love for my children to read it one day too