Hello,
At this age group, pinworms is a common cause of itchy perineum. For this, you need to get her stool examination done. Apart from that, it can be 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, medicines like some antibiotics and laxatives, anal abrasions and fissures and skin problems such as psoriasis, seborrhea and eczema.
All these causes need to be ruled out and then specific treatment started. Till then she can take oral antihistaminics which will help in her itching.
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.
A family doctor is great, but you could also see a pediatrician or an internist for a digital rectal examination. They go through a few more years of specialized training on this very subject. There could be a number of reasons that there's itching/pain/redness, ranging from pinworms to anal fissure to dietary sensitivity (acidic tomatoes for instance). I hope she finds relief soon!