Seems like you daughter has developed a pyoderma
on the back...
Pyoderma means any skin disease that is pyogenic. It refers to bacterial infections of the skin.
It is very important to clean it regularly with an antiseptic liquid daily and apply some antibiotic cream...Do not cover it with bandaid or anything...leave it open...but remember to wear only soft cotton clothes..If inspite of regular cleaning and application of the antibiotic cream, the lesion still is increasing in size,, then she might need oral antibiotics
...along with anti-inflammatory drugs...I would recommend that you visit a dermatologist
if the lesion persists or increases in size or if the child complains of excess pain or develops fever
Hope it helps
Dr Geetika Paul