Your doctor is right and I wish more doctors said this instead of treating pre-pubertal girls with an itchy bottom for candidiasis or thrush, which is a fungal infection that post-pubertal girls and women get, and is very rare in primary school aged girls.
Itchy bottoms however are extremely common. Pre-pubertal girls get something called vulvovaginitis and it is caused primarily by a lack of oestrogen at this age. They get red and sore around the opening to the vagina and, in extreme cases, the labia can get stuck together. It hurts to wee and there may be a small amount of discharge (which shouldn’t be smelly).
Emollient creams may help but antifungals are unnecessary. We can prescribe oestrogen creams in severe cases but it’s best not to use them for too long as they can be absorbed across the vaginal mucosa. Your daughter should wear cotton underwear during the day and no underwear at night. You should not use soap or shampoo in the bath. Try to avoid her wearing tights or tight leggings/trousers. The linked factsheet talks about putting vinegar in her bath (see below).
Threadworms are very common in primary school children; they look like short threads of white cotton and they come out at night to lay their eggs around the child’s anus. They can cause a very itchy bottom which can spread forward into the vulval area as the distance is not very great. Threadworms are easily treated with one dose of antiworming medicine from your pharmacist.
Occasionally children with a well demarcated, bright red area either around their anus or vulva have a streptococcal bacterial infection which is treatable with antibiotics. So you can see that there are lots of possible causes for your daughter’s predicament but thrush is probably not one of them.
There is a helpful Australian parent information leaflet at http://www.rch.org.au/kidsinfo/fact_sheets/Vulvovaginitis/