The pediatric dosage of Benadryl, or diphenhydramine, depends on the child's weight and the formula used, according to the St. Louis Children’s Hospital. For example, the recommended dose of liquid 12.5-milligram suspension is 3/4 teaspoon for children weighing 20 to 24 pounds.
Children weighing 25 to 37 pounds require 1-teaspoon doses of liquid Benadryl, while children weighing from 38 to 49 pounds require 1 1/2-teaspoon doses, notes the St. Louis Children’s Hospital. Children weighing 50 to 99 pounds require 2-teaspoon doses. Children 100 pounds or more should chew four 12.5-milligram Benadryl chewable tablets or take two 25-milligram capsules or tablets.