I do not study law, and thus may not give you the right suggestion. To my understanding, it is a big deal for an adult to assault a child, even your own your own child, let alone another person. You should call 911 immediately. You should certainly report to police, not for yourself, but for the whole community. We need to punish dangerous behavior following the law. It will be great if you can find witness to backup your statment. Also, you may think about bring your child to hospital to check potential hurt to him (mentally and physically), this might help you in the future ask for compensation (I am not sure about this, as I do not study law). It is just my 2 cents, but I really hate an adult attacking a child.