If your child is experiencing a rash after an allergy test, it is important to follow any instructions given by the allergist. If the rash is causing discomfort, you may consider giving your child Benadryl, but be cautious of potential side effects such as drowsiness or excitability. You can also apply fragrance-free moisturizing cream regularly to help soothe the skin. If you have any further questions or concerns, don't hesitate to reach out to a pharmacist for guidance.