Thank you for placing your query on Health Care Magic
Well you have diagnosed the disease and also discovered the treatment yourself. It is a condition called photosensitivity
which the child's skin is allergic to the ultraviolet rays in sun light. And the treatment is as simple as preventing exposure to sun light. Ask your daughter to keep most of her body parts covered when she goes out in sun and for exposed body parts use a sunscreen lotion with a good spf (sun protection factor) of at least 25 and above.
Hope that helps
God bless you