Just as when you are talking with a friend and you say something about another friends kid. you say so and so kid. ESPECIALLY if you don't know the spouse well, like someone you work with. Of course the kids belongs to both. But we are talking about the one person. Heck just yesterday someone said "did you hear Mark had a baby" hmmm I don't think guys have babies yet, but we all understood his wife gave birth, but they dont really know her.