Rash at which part of body?
ORS, syrup Zinc acetate, probiotics... These should be first line treatment.
Avoid milk till diarrhoea, give other solid foods, curd, butter milk, coconut water, lemonade, etc...don't give glucose water.
For perianal rashes, wash with water after every stool then dry it and apply Zinc Oxide cream.
Next Steps
if not relieved in a day or two or problem increases, consult Pediatrician in person.
Health Tips
maintain proper hygeine, frequent hand wash with soap-water, specially before meal.