I don't have a child who says that, but I've had a few kids say it to me at work. I usually say something similiar to Nerd- or say something like, well the rest of us are friends aren't we because we are all friends at xyz..... later I might take the child aside and explain how saying things like that hurt people's feelings and it isn't a kind thing to say.

It would really suck if it were my child saying it to me though, so I really sympathise.