Thank you for your query on Healthcare Magic.
The hives are due to allergic reaction
to some allergen like food, insects, clothes, and some times the cause is unidentified.
Check for anything causing it and if you are able to find out it just avoid its exposure.
You can apply calamine containing lotion externally over skin twice or thrice daily and you can give an oral antihistamine like benadryl
in prescribed dose.
Consult your Pediatrician
for the physical examination and for specific treatment.
Hope I have answered your query.