The dosage of Benadryl for a child is 1.25 mg of medication per kg of the child's weight, approximately every 6 hours. For a child weighing 9 kg, this would be equivalent to 11.25mg of medication. The children's product is typically concentrated at 6.25mg/5ml, so you would give 9ml of the solution every 6 hours if needed. It is important to reserve the use of Benadryl for rashes covering a large area of the body, and for localized or small rashes, a fragrance-free moisturizing cream like Aveeno is usually preferred. It is recommended to always consult a healthcare professional before giving Benadryl.