As per the description given by you the child seems to be suffering from Psoriasis and the blisters occur due to secondary infection after she scratches. Another possibility is herpes simplex
infection. In case of dry psoriasis, a mild steroid ointment( hydrocortisone) is sufficient. However as the blisters are recurring your child may need some local antibiotic
ointment and may be oral antibiotics
also. I shall advise you to consult a good Dermatologist
who can treat the child after examining the lesions.
Good luck and wish your child a quick recovery.