S has consistently been calling us Mr and Ms FirstName. She calls her bioparents Mommy and Daddy. She has regular visits with Mommy and occasional mail from Daddy.
So, we felt that the Mr and Ms FirstName was the best thing for her to call us. We attempted to introduce our mothers as Ms FirstName as well, instead of Grandma.
But she is three. And words like "Mommy", "Dad", and "Grandma" are not as emotionally loaded for a 3 year old as they are for the adults to whom they are applied.
And there are 3 other children in this house. And those children call me and Mr D "Mom" and "Dad". And they call their grandmothers "Grandma Lastname" when referring to them and just "Grandma" when in their presence.
So the grandparents quickly became "Grandma" and "Grandpa" to S, just as they are to Peter, Susan and Edmund. I told myself that was OK because most people have more than one grandparent and many children have more than two sets, so she wasn't really replacing "her Grandma" with these -- just expanding her circle.
Mr and Ms FirstName has stuck around, partly because our biokids make an effort to refer to us that way when speaking to her.
But, right this minute? I am at a McDonald's playground while my car is across the street being serviced. And S is climbing up to windows and hollering down to me, calling "Mom! Look at me!"
I don't know if "Mom" will last after we're back at the house. If it does, I hope that the difference between "Mommy" and "Mom" is significant enough for those to be different names for the two of us in her mind. I'm certainly not going to tell her she can't call me Mom, and yet.....I'm trying really hard not to take over her mother's place, although I am taking over that role in S's life for the time being.
It's a hard line to walk, much harder than I imagined when we signed up for this.