Thank you for your query on Healthcare Magic.
This itchy rash looking like an allergic skin rash
Allergy could be due to some plants or insect exposure.
in prescribed dose for itching.
You can apply calamine lotion externally two to three times per day.
Addition of H1 antihistamine like ranitidine
in prescribed dose will decrease the rash fast.
If the rash is persisting or increasing consult your Pediatrician
for the examination and for treatment.
Hope I have answered your query.