One father in our group apparently wanted to stir the pot. Early in the conversation he asked, "But how do we know what's right and wrong? Do those words have any meaning?""How about I punch you in the nose?"
"Don't do that - that would be wrong."
"So you do know. Not that hard now, is it?"
A discussion of what is right and what is wrong, and the meanings of those words and the implications they convey is a very good thing to have... amongst a group of people who are there for such a discussion and have a common base of understanding of basic philosophical constructs.
I would have told the gentleman that I would be glad to discuss at length his concerns at a later point, but to bring up such a serious and hypothetical series of questions was inappropriate for the setting.