Keep Your Cool
If it is your child on the receiving end of rude behavior, you might feel your temperature rise quickly. Before you react, stop and calm yourself. Going into the situation revved up and angry won't help anyone.

Your reaction will set the tone for the resolution.  Your child will look to you to see how they should react and the other child will be watching to see how effective their behavior was. Show them both that you can be unflappable in the face of rude behavior.