Thank you for your query,
This rash could be due to an allergic or a post viral diarrhea
Apply calamine containing lotion externally to decrease the itching and for soothing effect.
If the itching is more you can give oral antihistamine like cetrizine
in prescribed dosage.
If the rash is persisting or increasing consult Pediatrician
for further management.
Hope i have answered your query.