It seems that your daughter has developed a candidial infection of the nappy area. Candida is a fungus that often supervenes on a previous rash, especially if antibiotics have been used on it before, either locally, or through the oral route.
You would need a locally effective cream of antifungal medication such as clotrimazole
. Apply one of these two or three times a day and keep the area dry between soilings. She will recover soon.