Pediatrician advised to give my 21 month old 24 pound daughter 1 tsp dimetapp cold and cough childrens. is this okay?

Old medication: This is a product sold by prescription 3 decades ago but available over the counter for the last 2. It underwent a formulation change about 10 yrs ago, reducing its strength. At about the same time most realized these products have little value and advised against use under 6. I find this dose a little heavy, by double (when used for allergies).