It depends on the child's weight The dose is about 0. 5 mg per pound of bodyweight every 4-6 hours. 5 ml (1 tsp) of liquid Benadryl has 12. 5 mg. A 50 lb child could have 25 mg (50lb X 0. 5mg/lb = 25 mg). This is 2 tsp of liquid.
For child below 12-months old, consult your doctor. Increase the teaspoon's level (start by 1/4 tsp) by quarter from first year for every 2 years. Dosage depends on weight too.