There are many potential causes of arm rashes in a child. However, in the setting of developing this rash on sun-exposed areas, solar dermatitis is the most common diagnosis.
Solar dermatitis (or sun rash) can have many different causes. Most commonly among children, this is idiopathic (no clear cause), and can be avoided by avoiding exposure to sunlight or a higher SPF level of sunscreen.
There are other causes or exacerbating factors for sun rash. Plant products or chemicals (including creams or perfumes) can worsen sensitivity to sun. Also, antibiotics such as tetracycline can cause a photosensitivity. Very rarely, sensitivity to sun can be part of autoimmune diseases such as lupus, but this would be highly unlikely in a child. Finally, there is a rare form of allergy to sunlight called "solar hives
" or "solar urticaria" that can be an allergic-type reaction to sunlight, but this is also exceedingly rare.
Solar dermatitis in and of itself does not pose any long-term harm, but excessive sun exposure at a young age can be harmful. Ultimately, avoidance of the sun, increased sunblock, and evaluation by a dermatologist
are most likely to diagnosing and helping to treat and prevent your daughter's condition.