Hello,
It can be due to contact dermatitis due to 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, certain food irritants, seborrhea, eczema or fungal infections.
Since the child is young, I suggest you to get an evaluation done from a dermatologist as treatment is diagnosis specific.
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.
I do have a picture, if that is allowed to be uploaded...