six year-old boy was pushed down and arm was broken

My son,a first-grade, six year-old lovely boy, was pushed down, by a first grade American girl in school during a recess last Thursday when he was playing a monkey bar, and his arm was broken. I talked to the school's principle. He did not indicate any responsiblity. He even did not inform the parents of the little girle. I am very sad to see my boy in pain. Friends of my wife suggested to sue the girl's parents or/and the scholl. We do not have any legal experience in US. Does anybody know how to deal with this case? Please give advices. Thank you in advance.