To calculate the dose of Benadryl for a child, you should give 1 to 1.25mg of the product per kg of the child's weight, every 4 to 6 hours for a maximum of 4 times per day. For example, for a child who weighs 12kg, this would be approximately 12mg four times a day. The solution for children is usually concentrated at 6.25mg/5ml, so this would be around 9ml for a 12kg child. For a child who weighs 23kg, the dose would be 18ml.