I think you could use a second opinion, it could be an infestation and your child needs treatment, it could also be an allergy, what does she bathe in, if this came on suddenly has she had any new clothing she may be allergic to.What is her health like in general, eating okay any fever?
Itchy rashes "all over the body" are sort of rare, carce. Usually rashes are localized to specific areas, like sweaty area such as armpits and backs of knees or whereever skin folds over, or to the back, or hands and feet, etc.
Can you actually see a rash on her, or is she just scratching at some itch that you can't actually see?
Does it look like it's possibly insect bites? Did it start in one specific location?