I usually called them "Mom" and "Dad," but then I and another boy got lost in the grocery store at the same time. With both of us wandering around yelling "Mom," one of the moms followed the wrong voice, and went up to the other mom's child. I heard them joking about it afterwards, and got the misimpression that the "Mom" and "Dad" labels were confusing, so I called my parents by their first names for a few days or weeks.