Well, my daughter has no memory of her biological mother and my son.. he remembers some but exactly what he remembers, I'm not sure. My mother tried to pray for her one night when she tucked them into bed while we lived with my parents for a short time a year ago and my son told her, "No! Don't pray for her..."
I've never spoken much of her at all but one time, a few years back now, some commercial on television sparked something and my son turned to me and said, "Daddy... I wish we had a Mom." I was very unsure of what to say because it was so unexpected, but I ended up spitting something out similar to this: "Well son... sometimes, both parents just aren't good or aren't good together..... So, don't you think it's a good thing to have one good parent instead of two who are always arguing and being mean to each other?" He thought for a moment, then he smiled at me and gave me a huge hug.
I decided a long time ago that I would never speak badly of her to either of them. I figured I'd let them decide for themselves as they grew up what they thought of her on their own, and as they get older and ask more and more questions, depending on the age and maturity level at the time I will answer everything as truthfully as possible.
I have stayed in contact with her family for their sake and I make sure that they see them as often as possible. I told them that my only "rule" was to never speak of their mother, and they have always abided by that. (She has no contact with her own family and hasn't for years now, too.)