Thank you for your replies. My main question is what is the risk of peanut cross contamination and how does it occur for M&Ms.
The reason I wondered about it was to help me figure out what the risks were compared to my craving for them. If the risks of allergic reaction to having them around was low because cross contamination levels were low, then it would be acceptable to me to have them in the house. Don't know if I'm expressing myself clearly here and really sorry if my muddled writing isn't getting the point across.
I think I'll try to call Mars Inc. after the holidays and get some more info - I can't remember the last time I spoke with them but it may have been 9 years ago so maybe they have had more training. If I get any useful info I'll post back here.
With respect to my kid's feelings about having candy around that he cannot eat, he regularly sees us not eat sweets that he is enjoying - not because we are allergic, but because we are trying to eat more healthy foods, or just don't feel like eating sweets everyday. He does not feel that we all need to eat everything that everyone else is eating at dinner (e.g. he and his dad won't eat the beets, brussels sprouts, cabbage, and spinach that I frequently cook and eat, and I don't eat the ceasar salads that I make for them several nights a week). After I started eating some of his safe Halloween candy he asked me not to eat any more of it and I said that the only type of candy I was craving was M&Ms which weren't safe for him. He said I should go out and buy those and eat them - anything to protect his Halloween stash! That's when I told him I'd try to investigate whether they'd be safe enough to have in the house.
With respect to having his home be an allergy free zone - we do not have his allergens in the house as a "listed ingredient" although we do have a very few "may contains" or "on shared equipment" products that I handle almost as if it were toxic. I encourage him to read every label every time. I don't want him to learn the habit of going into a cupboard and just grabbing stuff and eating it without checking labels first. I do this because I hope and pray he'll be able to live with other people (maybe go to college, maybe get married and have kids) and then he'll be sharing a kitchen with others so he won't be in control of every food that comes into the house.
Thanks again for your replies - just thought I owed you all an explanation for my question.