Urticaria due to allergic reaction: This is the most common cause of urticaria in children, often triggered by food, insect bites, or environmental allergens. The migrating nature could be due to the spread of the allergen or the body's immune response.

Insect bite or sting reaction: Reactions to insect bites or stings can cause urticaria that may appear to migrate as the body responds to the venom or saliva injected by the insect.
Viral exanthem: Certain viral infections can cause urticarial rashes that may seem to migrate as the rash evolves.
Autoimmune disorders: Conditions like autoimmune urticaria can cause recurring and migrating urticaria due to the body's immune system attacking its own tissues.

Mastocytosis: A condition characterized by the accumulation of mast cells in the skin, which can cause severe urticaria and anaphylaxis. Although rare, it's critical to diagnose due to its potential severity.
Serum sickness: An immune reaction to certain medications or antiserum, which can cause urticaria among other symptoms. It's essential to identify and manage promptly to prevent serious complications.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: Although these conditions are more severe and less common, they can start with urticaria-like lesions and are medical emergencies due to their potential for significant morbidity and mortality.

Erythema multiforme: A skin condition characterized by target lesions, which can sometimes be confused with urticaria. It's often associated with infections or medications.
Urticarial vasculitis: An inflammation of the blood vessels that can cause urticaria-like lesions. It's a rare condition that can be associated with systemic diseases.
Parasitic infections: Certain parasites, like hookworms, can cause urticaria as part of their infection process. This would be more likely in children who have been exposed to contaminated soil or water.