My daughter is 4 yr old 48 lb they told me to give Benadryl (diphenhydramine) she ha parainfluenza to help calm cough how much to I give her?

Benadryl (diphenhydramine) dose. the dose for Benadryl (diphenhydramine) is 5 ml for every 22 lbs of weight. So for your daughter, it's about 10 ml, or 2 teaspoonfuls every 4 to 6 hours.
The Benadryl (diphenhydramine) dose. for a 48 lb child is 25 mg every 6 hours (2 tsp of 12.5mg/tsp) or 2 chewable of 12.5 mg chewables.