A sitz bath is a warm, therapeutic bath used to heal and cleanse the area around the perineum (the space between the genitals and anus). It can be done in a bathtub. Soaking in water warm water on its own can help speed up the healing process by boosting the blood flow to the affected area. It won't cure the condition, but it can help soothe the irritation. A simple option is to use your bathtub. Fill the tub with 3 to 4 inches of warm water and slowly lower child  in sitting position, making certain the temperature is warm enough to be therapeutic but not scalding. U can add baking soda or vinegar but no soaps or detergents.

If condition doesn't improve she should be evaluated by pediatrician