Of course it's a little girl. They couldn't have a proud, defiant adult woman standing there, facing down the bull. That would be too much.
Observe the sexual contrast: The bull, an adult male, powerful and wild, symbolizes masculinity. The girl, pre-pubescent, defiant, brave, but ultimately small and weak, is womankind. Typical condescending patriarchal sexism.
Just kidding. I kind of like it, actually. I prefer to think of the bull as signifying the markets -- a big, powerful, dangerous but ultimately domesticable animal -- being faced head on by the girl (humanity), who is young and naive and probably doesn't understand the danger. She's immature enough to think she can handle the deadly beast. She's probably wrong.