Phototoxicity/Sunburn: This is the most likely diagnosis given the description of a sunburn-like rash in a child without fever. Prolonged exposure to sunlight, especially in fair-skinned individuals, can lead to sunburn, which presents as a painful, erythematous rash.

Contact Dermatitis: This could be due to exposure to allergens or irritants such as plants (e.g., poison ivy), chemicals, or certain metals. The rash can appear similar to sunburn and is often itchy.
Insect Bites or Stings: Reactions to bites or stings can sometimes mimic sunburn, especially if there are multiple bites.
Heat Rash (Miliaria): Although more common in warmer climates, heat rash can occur in anyone who has been in a hot environment, leading to blocked sweat ducts and a rash.

Stevens-Johnson Syndrome (SJS): Although rare and usually accompanied by fever, SJS is a severe condition that can be triggered by medications or infections. It starts with a sunburn-like rash but can progress to involve mucous membranes and is potentially life-threatening.
Erythema Multiforme: This is an immune-mediated condition that can be triggered by infections or medications. It presents with target-like lesions and can sometimes resemble a sunburn-like rash. While not always severe, it can be a sign of an underlying issue.

Pellagra: A condition caused by niacin deficiency, which can lead to dermatitis, diarrhea, dementia, and potentially death if left untreated. The dermatitis can resemble sunburn.
Porphyria Cutanea Tarda: A disorder that leads to blistering of the skin upon exposure to sunlight, which can sometimes be mistaken for a severe sunburn.
Polymorphic Light Eruption (PMLE): An allergic reaction to sunlight that can cause a rash, which might resemble sunburn but is usually more itchy and less painful.