I have a 13 month old with a cough and congestion. I have some prescription cough medicine?

Does anyone know about how much I could give my son. My daughter was given 1 teaspoon every 6 hours. It is called Carde DM Syrup substituted for rondec dm syrum. I know I shouldnt give meds without it being prescribed to them but i am short on money right now and I know this worked great for her. she is about 85lbs and he is 26lbs. Please serious answers.