Suggest Treatment For Sore And Peeling Skin On Buttocks

My daughter was complaining of a sore bum on Saturday and she went to the washroom when I was not looking and lathered her bum in cream - the nistatin cream was open (which was prescribed to my son for a yeast infection|) - I am not sure if this is what she put on her bum, but I wiped it off and then put a diaper rash pentanen cream on her - today her bum is peeling as if it had been burned - what should I do?