Almost certainly a hematoma, especially if she was pinched by something. It probably won't hurt her at all however vetinary attention is needed for treatment. As the chest area cannot be bandaged it is diffucult to treat yourself and you don't want it converting into a fibrous mass which can sometimes happen.
Also, cold hosing/ice packs are very good but are mostly effective in the initial stages as it can reduce the swelling as the blood vessels begin leaking but once the hematoma is formed it will have little effect.
All horses deserve, at least once in their lives, to be loved by a little girl.