The soap and water are fine, if followed by the plain Neosporin or Triple Antibiotic Ointment. Chlorhexidene or betadine can be found in most pharmacies and in feed stores. Hibiclens is a pharmacy brand of chlorhexidene and Walmart has it as well as drug stores.
If you decide to treat it, I would bring her inside in a dog crate with food and water, and on a soft towel for bedding. Then once a day I would soak her bottom in warm Epsom salts or warm water with some chlorhexidene or betadine, and gently rub the area before rinsing. Then apply plain...