I never outright told my parents anything regarding my position. I think they know, since they once stopped by my house and I forgot that I had a few decorations out, and have had it confirmed since my son is also vocal about his birthday or christmas gifts. They never say anything to me about it, and have never berated my son for saying anything about it or tried to tell him that it is wrong. I do wish I could have been a fly on the wall when my son told them at dinner when he was visiting them one evening that he had "seen Santa and he's real!" That little tidbit came courtesy of my brother, who was there. I decided a long time ago that it is my decision to raise my child as I see fit and that I would not try to prevent my son from talking about things that are important to him.
It has been a long time since you went to meetings, your parents probably already at least suspect. Unless you feel a burning need to test the waters, I would say just let it go and live your life. If they are curious because of something your son says, they will ask or maybe you will be lucky like I am and they will just play the don't ask, don't tell game and you can live in peace without the confrontation.