It was at my daughter's birthday party at the nursery. Seventeen two-year-olds sat on little chairs in a circle and Darling Daughter (DD from now on) sat on the birthday throne. I sat next to her and waited eagerly for the teacher to begin - it was my first time. "Who's having a birthday today?" asked the teacher? "DD!" shouted the children (except that they used her real name of course). "And who has come to celebrate her birthday with DD?" "DD's mummy!" "That's right." Before she could continue a little voice from stage left called out, "where's daddy?"
I turned to the little girl and stetched my hands out, palms upward, with a look of I don't know, where could he possibly be? on my face, and we moved on. DD didn't even notice. As yet she has no idea (as far as I know) that our family of two is in any way unusual. However, this event has given me a 'heads up' that we are heading into question territory and it's possibly just around the corner.
So I came up with a line that we will use until more detailed explanations are appropriate: "DD doesn't have a daddy, but she has a mummy who loves her as much as a mummy and a daddy together!"