Maybe dunno could offer an opinion on the wisdom of what I'm going to suggest........
Give her the weekend to allow the likely bruising to diminish. The place two chairs facing back to back about arm width apart. Have her stand between the chairs and put one hand on each chair and gently press down on both chairs as if she were trying to do an iron cross like the guys do on the rings. If she has pain that's centered in her sternum, like straight up the middle, the problem likely goes beyond superficial bruising. and may involve cartilage bruising or strains that need more than minimal time to heal. If the pain is only felt along the muscles that radiate away from her sternum, her recovery is probably a matter of working within her own level of pain tolerance...... which varies greatly from child to child.
The advice to go to a sports med doc is good, and you should ask around to se if anyone you know can suggest one they've used. If she doesn't need a sports doc now, at least you'll have a name to start with in the future.