If they asked, I would let them know my views. I would express my honest opinions to them if they offered their views to me. I would owe them nothing less.
I may explain my opinion, but if they don't buy it, they don't buy it, they live on as Christians and I live on as an Athiest, I might not LIKE the fact that they became Christians, but there's only so much I can do.
I am not an Atheist but I am a Satanist. I would just tell them to the best of my knowledge what is true and not lies. I would let them live their own lives how they please not not force them into anything that they are not willing to believe.
I wouldn't care either way considering about half of my family is Christian. I probably wouldnt really say anything to them about their beliefs, and if i did i would refer back to knowledge that i do have from when i was christian. I would not try to convert them. If they did convert then great, and if not then great.
My son has expressed interest in Christianity. I try to give him support, but also all the available options so that he's better equipped to make choices.