Antihistamines are a part of many cold medications to reduce the nasal discharge. They have an added benefit in suppressing irritant cough. They may cause constipation as a side effect. Make sure you plan the child’s diet accordingly. If there is an infection, the cough will not subside till the antibiotic starts to work and it will continue for a few days even after fever subsides.
Finally, I’d like to say that cough prescriptions should not exceed a dosage period of more than two weeks. If a cough does not subside by then we should look for chronic conditions in and outside the lungs.