Urticaria (Hives): This is the most likely diagnosis given the rapid onset and resolution of the red, raised rash. Urticaria can be triggered by various factors, including allergic reactions, infections, or even physical stimuli. The fact that it resolved without intervention within an hour is consistent with acute urticaria.

Insect Bites or Stings: These can cause localized red, raised rashes that may resolve quickly, especially if the child was exposed to the outdoors.
Contact Dermatitis: Although less common in such a short timeframe, contact dermatitis from an allergen or irritant could cause a similar presentation.
Viral Exanthem: Some viral infections can cause rashes that come and go quickly, though they often have other accompanying symptoms.

Anaphylaxis: Although the rash resolved quickly, any sign of an allergic reaction, especially if it involves multiple body systems (e.g., respiratory, cardiovascular), should prompt consideration of anaphylaxis, a life-threatening condition.
Meningococcemia: This is a severe infection that can cause a rash, often described as petechial or purpuric, which can evolve rapidly. It's crucial to consider this diagnosis, especially if the child shows signs of severe illness or if the rash does not follow the typical pattern of urticaria.

Kawasaki Disease: An acute febrile illness that can cause rash, among other symptoms. It's rare but important to consider in children with unexplained fever and rash.
Erythema Multiforme: A skin condition characterized by target lesions, which can be triggered by infections or medications. It's less common in young children but should be considered with atypical rashes.
Acute Hemorrhagic Edema of Infancy: A rare condition that presents with a distinctive rash and edema, often on the face and extremities. It's self-limiting but can be alarming in its appearance.