Sorry to hear about the accident.... Since she has developed a fever, I´d like to hold back with my recommendations. However - one thing that comes to my mind and it M A Y help is rubbing the arm that hurts with castor oil a few times a day.
Also Arnica Montana D6 (homeopathic medication - D6 is a very low potency) may help... It has no side effects and is safe for anyone big and small. It usually comes in globuli...small Sugar pills. 2 or 3 of those should´nt do any harm, I imagine. BUT use at your discretion. Arnica Montana is a great homeopathic remedy for aches and pains of all types, muscles or bones - pl google for more info. I know some mothers who carry Arnica around in their purses and whenever their kid has a fall or whatever they give them Arnica... but as I said, since she has fever, it may be something more serious - in which case you´d need to go see a child´s doctor.
Best wishes for GOOD health!