I think her 3 year old son is right on the mark:
Charlie’s processing of the relationship started at school. “His teachers were just so great about it,” Nixon says, “because they were the first people that started referring to ‘Charlie’s moms,’ which is so lovely, and we really hadn’t done that yet. So Charlie came home one day and he said [to Marinoni], ‘You’re my mommy too!’ ” Deciding to seize the opportunity, Nixon began calling her partner Mama Christine. “I said, ‘Charlie, where’s Mama Christine? Is Mama Christine in the other room? Would you take this to Mama Christine? What does Mama Christine want for breakfast?’ I did this a nauseating amount, and one morning we’re at breakfast and Christine is in the shower and Charlie says, ‘Where’s Papa?’ My daughter says ‘Papa? You mean Daddy?’ And he said ‘No, Papa! Christine!’ He’d come up with this masculine name for her. It’s gone through a whole series of things.” When Charlie’s Russian barber told him to ask his daddy to give him a little brother, Nixon says she stood back to watch the preschool-age Charlie’s *reaction. “I was just going to let it go—what are you going to say? But I saw Charlie—I saw it land and how he thought about this, and he was quiet for a while. Then he very slowly said, ‘Sometimes I call Christine Mommy.’ It was like Charlie was trying to navigate what everybody’s assumptions about him and his family were at age 3. It was so amazing.”
http://advocate.com/printArticle.aspx?id=110591
She's Papa and she's Mama Christine and she's Mommy
