Lots of things can cause rashes. Heat, an alergic reaction to clothing, lotion etc. even constant rubbing or hand contact. Hydrocortisone will work but its not good for broken skin to make contact... if the rash is open. Best to get it diagnosed to be safe, because you may not know whats causing the rash and are likely to re-expose it without knowing the cause.
oh well if its poson ivy then use calamine lotion and apply it using a cotton ball. do not touch it or wash it and make sure no one uses the cloths or washes their clothes with her things and don't let her scratch or rub it it spreads