It can be due to contact dermatitis, allergies, moisture(due to sweating or frequent washing), the abrasion caused by clothing,the pressure of sitting, chemical irritants like laundry soaps, colognes and scented toilet paper, due to the dermatitis by the bleaching powder or due to skin diseases like eczema or seborrhea.
First of all stop using the bleaching powder as it may be further aggravating the rash. Avoid using any lotions and creams for the meantime. Wash your area with a mild cleanser with moisturizing properties like Cetaphil. Pat it dry. Then apply a good hypoallergenic moisturizer over the area and see if this improves. If the symptoms still persist then you can apply mild steroid and make her wear cotton nappies.
If the symptoms are severe then please do consult a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.