They appear suddenly, and move from place to place within hours. They itch intensely. There are many causes. You may want to keep a log of potential exposures such as food, bug bites, etc., and determine if you find a suspicious pattern over time. An episode of hives usually lasts 1-2 days, but rarely, can go on even as long as several weeks.
Most children have only one episode of hives, but some have many.
Benadryl is the dose of approximately mg per pound of body weight every 6 hours. It is most effective to give the antihistamine at regular intervals, rather than wait for the hives to reappear. You will usually need to continue the anti-histamine for a few days. This parent education document is a guideline only. You, as parent or primary care giver know the child best.