You should really take her to her doc to see if it is a yeast infection before treating at home. Once it's diagnosed, your doc can tell you proper prevention steps. It could also be pinworms - which yeast infection treatments won't help. Most yeast infections also cause discharge, so if she's not having any, it is likely pinworms or contact dermatitis.
Some tips are to make sure she's wearing cotton panties and that her pants aren't too tight (such as no leggings). If you aren't using a dye and perfume free detergent, you should switch and she should also refrain from taking bubble baths or using scented body wash. If she's been going swimming, she needs to change out of the wet swimsuit right away and not after a car ride home.
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