Viral Exanthem: This is the most common cause of hives in children, often associated with viral infections. The six-day duration fits within the typical timeframe for many viral exanthems, which can cause urticaria (hives) as part of their presentation.

Allergic Reaction: An allergic reaction to food, medication, or an environmental allergen could cause hives. Given the duration, it's possible the child was exposed to an allergen that they continue to be exposed to or that has a prolonged effect.
Insect Bites or Stings: Repeated exposure to insect bites or stings could cause ongoing hives, especially if the child has an allergic reaction to them.
Autoimmune Urticaria: This condition involves the immune system attacking the body's own tissues, leading to hives. It can be chronic but might start acutely.

Anaphylaxis: Although anaphylaxis typically presents with more severe and immediate symptoms, it's crucial to consider, especially if the hives are part of a broader allergic reaction that could escalate.
Mastocytosis: A condition where there's an accumulation of mast cells in the body, which can lead to severe allergic reactions, including hives. It's rare but critical to diagnose due to its potential severity.
Serum Sickness: An immune reaction to certain medications or antiserum, which can cause hives among other symptoms. It's less common but can be serious.

Urticarial Vasculitis: An inflammation of the blood vessels that can cause hives, but it's much rarer and typically associated with other systemic symptoms.
Thyroid Disease: Both hyperthyroidism and hypothyroidism can cause hives, though this is less common and usually accompanied by other symptoms related to thyroid dysfunction.
Parasitic Infections: Certain parasitic infections can cause hives, though this is rare in many parts of the world and would typically be associated with travel history or specific exposures.