I have a family member who was sarcastic and indifferent to a passenger who asked her to stop her son from kicking his seat. I wasn't on that flight, but when she relayed the story to me, I surprised her by saying how wrong she was and how the other guy was in the right. Parents have an absolute responsibility to do everything in their power to keep their kids from acting out on an airplane.

I would start with the parent and then escalate to the FA if for no other reason than to hope that embarassment may motivate the parent more than my admonishment.