My suggested response is: "Mu" & let them deal with figuring out what that means.
To paraphrase the science fiction author Robert Heinlein: To argue with them would be like trying to teach a pig to sing; all it would do is frustrate the teacher -- and annoy the pig.
There is a good reason that the Buddha did not spend time with answering some metaphysical (the Unanswered) questions. Spending time in such pursuits does not move one forward in the path to enlightenment. Neither will your spending time answering such taunts. Best
As to your duty to bring them wisdom which will lead them on their individual paths to enlightenment, remember that the Buddha said it is only to the willing should you make such efforts. It does not look to me that they would be willing and you have no responsibilities here either.
Best to you, Jim