My Pediatrician and our local pharmacist recommend no cough syrup before the age of 6. The reason is that in a small amount of cases it has caused swelling of the brain. If you look at the label on cough syrup for children, even those typically recommend for age 6 and up (at least the brands I have seen.)
When my 2 1/2 year old has a cold with a cough I give him milk slightly warmed with honey in it. I have also given him a teaspoon of honey on it's own. It works extremely well.
I have copied this out it may be some help, it may be a good idea to ask your doctor or pharmacist....be careful with cough syrups
If your C-Phen DM is a generic form of prescription Rondec-DM (i.e., with antihistamine chlorpheniramine 4 mg, decongestant phenylephrine 12.5 mg, and cough suppressant dextromethorphan 15 mg, each per 5 mL), then the guideline dose for a 4-year-old is one-quarter teaspoon (the one-half teaspoon dose is for age 6-11 years, while the full teaspoon dose is for ages 12 years and up).