So hard to tell with leg injuries over the internet! Sounds like she has kicked out, and connected, with something hard and sharp. I know I seem like I am stating the obvious here. What might actually be causing the swelling is not the abrasion itself but the impact of the kick with whatever it was she connected with.
I have seen horses that kick out and hit a barn door, fence, another horses hoof and sustain a nice swollen fetlock/canon bone area as a result. Cold hosing will help for sure and you may be doing that anyway as you clean the cut.
Has it gone down at all in the last two days? Any change in appearance, heat, hardness of swelling? I would keep an eye on it for all those changes.
All horses deserve, at least once in their lives, to be loved by a little girl.