Thank you for your query,
The cause of Hives is allergic and the culprit of allergy
may be external like pollen, animals, plants, fish,egg,oils and some nuts and internal like worm infestations.
Most of the times detecting the cause for allergy in a particular child is difficult and requires the making a diary of daily food, activities and correlating the symptom with them.
For itching you can use long acting antihistamines like fexofenadine
after consulting your Pediatrician
and applying calamine
Hope i have answered your query.