Thank you for your query,
It is looking like Allergic skin reaction with Urticaria
The cause of allergen may be some change in food, exposure to pet animals, pollen or exposed to clothes containing allergens.
Most of the times it is difficult to find out the cause and a diary should me maintained about child activity,diet,exposures to pollution and animals as it will help to find out the allergen by analyzing the history.
You can apply calamine
containing lotions over skin for two to three times daily and oral antihistamines can be given in prescribed dosage to decrease the itching.
Consult your Pediatrician
for evaluation and medication.
Hope i have answered your query.