They super imposed her head onto a younger girls body and they use an airbrushing technique..
Yes to the airbrushing, no to the super imposing of the head. That's her own body, and she does the dancing and everything else.
Here's an image from the making of Benjamin Button coffee table book, you can see her test fitting her dancer costume. Click to enlarge.

As you can see, it's her own body, and it looks simply smashing.






There was a stand in, but just for a couple shots that required exceptional skill.









something I think only Cate with her stature and mystery could pull off as well.








