It is the face that a girl makes when you do one of two things
A. Do something that is very kind and they didn't expect it.
B. You make fun of them and they don't really mind.
The face it's self is a slight tilting of the head to the side, the eyebrows arch in and a covering of the lower teeth by the bottom lip. The main distinction between A. and B. is that A. is followed by a short,"Aww." Whereas B. is followed by a coughing sound.
A. After I gave her the rare Moody Blues CD that she wanted, she made the girl face.
B. When I told her women shouldn't be allowed to vote she made the girl face.