What a great shot. Such a look of concentration combined with a smirk of satisfaction. I like the crop - letting the missing hand lead you into the photo. You may want to do some PP on her legs - looks like she scratches mosquito bites like my daughter Christina.
Great composition! I love the light play on the floor. I have a daughter in Ballet so I know how difficult it is to work with the lighting in the practice rooms. Makes me want a 24-70 2.8L!