Oh I love this!
For DD, we started out calling it nursies. Once she started talking though, she had her own lingo, and it was deemed "Boob," so whenever she wanted to nurse she would say, "I want boob," and that was that. It really was cute and I wouldn't have changed it for anything. If she fell and got hurt she would cry, "Wahhhh....beeeeewwwwbbb!" and I would just scoop her up and nurse her. Amazing to remember!!
Now, since she's getting a crash course in reproductive anatomy and learning appropriate terms (funny story about her telling DH's brother that baby was "in Mommy's belly until it comes out her 'gina") she has since learned the term breastmilk and I LOVE hearing her say it. She'll tell anyone and everyone that babies drink "bressmilk", we drink soymilk, and some people drink cows milk (b/c we're vegan) and it just makes my heart swell. LOL! It's the little things.