Keeping Children out of Conflict
The issue of whether to call a step-parent "mom" or "dad" is difficult for some families. Let the child determine how this goes. Never push a child to call a step parent by these names.
This facebook entry brings up how my son and I handled this situation. We talked about it!
The first thing I wanted to do was to find out what DAD meant to my son and to tell him what it meant to me. We had to redefine what DAD meant. This allowed us to understand each other better. It gave him the control and confidence as we both knew what he meant when he called me DAD.