Why is my 6 year old daughter's vagina itching?

Could be. Vaginal itching in a 6 year old can be caused by a reaction to chemicals like bubble baths, pool water, etc. Pin worms can also cause rectal itching which can be hard to distinguish from vaginal itching. Children this age can put foreign bodies in their vaginas just as they do in their ears and noses and this can cause itching. Her pediatrician will be able to tell you what it is in her case.