I think this is a great, energetic name. I would use it as a nickname for my grandmother's name, Pauline, which I can't really see on a small child (she didn't like Pauline either, and opted to go by her middle name, Evelyn).
Polly is cute, traditional, and strong, and it doesn't care the babyish image that Molly does.
Polly Jean : like Polly Jean Harvey who I admire. It sounds powerfull, yet feminine and kind of sweet.
Polly started out as a medieval diminutive form of Mary, through Moll/Molly. See Also: Mary, Molly