I just have a few tips and stuff, but yeah its all for you to filter out :P

1) I understand that your concerned for your son. I know that at church and the Bible, it teaches that What does light have to do with darkness? and were not suppose to yoke with non believers. but just remember that when jesus came on earth, he sat,talked, and ate with the non believers out of love, and to reach out to them, he could have sat with his disciples, but that wouldnt be true love, and God came for the sinners, not for the rightous.

2) Of course we cant asume that EVERY person on earth belongs to a church, so i think that question u asked was really out there and random, that could have made her feel really uncomfortable. Just because a girl/family is an athiest, doesnt mean they will remain that way. YOUR son could have been the light for her, and her family if he is spiritually strong with God.

3) what u did was wrong about snatching the cookies away etc.. honestly i found that kinda funny, but u know that would hurt her feelings. were u born christian? we were all non believers at one point. let this be a chance for evangelism. you really should apologize to her for what you have done, because think of it this way, your representing a christian. what do u think her views would be of christians now?

4) pray to God, ask for forgivness. talk to your son and ask for his forgiveness, and u should invite her to dinner or something to make it up, and dont bring up " religion" to the dinner table unless she brings it up. all religions are MAN made, Love is what makes the difference.

i hope this helps