A cough suppressant alone would be better. If nasal congestion is the major problem, consider using a decongestant nasal spray rather than an oral medicine. This way the medicine is going right to where it is needed, and there are fewer side effects. The spray should only be used for 3 or 4 days. Only give the cold remedy for 2 or 3 days, when the symptoms are at their peak.
Prolonged use can lead to more side effects and complications.
If your child has chronic respiratory problems such as asthma or cystic fibrosis, you should talk with your doctor before giving cold medications. Only give the dosage that is recommended on the package. More is not necessarily better. The dosage is usually based on your child's weight, and excessive amounts can cause greater side effects. There are other things that you can do to make your child feel better during a cold that don't involve buying medications.