Thank you for your query,
This rash could be Allergic rash
as it is itchy and there is a preceded history of probable exposure.
You can apply calamine
lotion externally over rash for soothing and to decrease the itchy sensation.
If there is still itching oral antihistamines can be given, so consult your Pediatrician
whenever he is available if rash increases or if it doesnt subside within 24 hours.
Hope i have answered your query.