Viruses of many types can trigger hives as well. They were hypersensitive to something, but no one discovers what, and the hives go away. Recent thinking is that most of these unidentified cases may be from viruses, making them the most common cause of hives. Hives are usually treated with an antihistamine, such as Benadryl or the prescription Atarax. If drowsiness is a problem, one of the newer non-sedating antihistamines may be a better choice.
Stronger treatments are available, but are usually unnecessary.
If there are other allergic symptoms, such as wheezing or tightening of the throat, then a shot of epinephrine may be needed. Usually the whole thing resolves fairly quickly. If the hives keep coming and going for six weeks, we call the condition chronic urticaria. This is often not serious, but can be a sign of an underlying disease process. These children should have a thorough physical and lab work, to be sure.