The most common cause of hives on a child's skin is an allergic reaction, as stated by Kids Health. An allergic reaction can be caused by an insect bite or sting, some foods, pollen, some medications and pets.
If the hives are not caused by an allergic reaction, there are several illnesses and other things that can cause them. Some of these causes include an infection, stress, scratching, sunburns and excessive pressure to the skin for prolonged periods of time, as stated by Kids Health. In most cases, it is very hard to pinpoint the cause of the hives and they usually go away on their own in a few days.