I read somewhere that you shouldn't call them anything you wouldn't want them to say in public... so I just say, "do you want some milk?" He isn't talking yet so hopefully when he does start talking he will just say "milk" and when he can form sentances, hopefully he will just say, "I want milk"
I dunno.. time will tell. But I don't use any nickname for them other than "mama" and "milk" That way when he is older I won't get weird looks, like if he were to say in front of strangers, "I want boobies" or something
My BFF calls them "titties" I never thought that was very appropriate, but she didn't care, so that's all that matters.