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Looks like your grandson has developed a lesion of Vector dermatitis..also known as Paederus dermatitis
Paederus dermatitis is a peculiar, irritant contact dermatitis
characterized by a sudden onset of red lesions on exposed areas of the body. The disease is provoked by an insect belonging to the genus Paederus.
You need to apply an antibiotic/steroid
combination cream over the lesion (something like flutibact, fucibet) twice a day.
Take oral antihistamine like levocetrizine once a day to decrease the swelling (Dosage according to age of the patient)
Do not use any moisturizer or oil over the lesion
Do not cover the affected area
Wear cotton clothes
Hope it helps
Wishing your grandson speedy recovery
Dr Geetika Paul