I would absolutely step in in this situation and would expect other parents to do the same with my child. While I would never shout or scream at another person's child, I would talk with the child and try to correct the situation. Now, if it is a playful aggression and a case of "boys will be boys," then keep an eye on it, but no reason to overreact.
What do you want the panelists to weigh in on? Email your ideas to editor Tamara O'Shaughnessy at tamara@chicagoparent.com.