Persistent cough. What should i do?

Last 3 months my 4 year daughter is on cough syrups. before that she had viral pneumonia. whenever she starts coughing it last for months. I tried homopathy and allopathic both treatments but still whenever she develops cough it last for months. she develops cough very frequently, bimonthly. What should I do? I changed 4 doctors because she is not improving. I also have a fear that because I change dr and they change medicines, the virus or infection may devlop resistance against medicines. some dr says she has bronchitis asthma some just give medicines. 1) is there any way by which exact cause of cough will be known. 2) Does cough syrups impacts kidney or any other major side effect as she is consuming them from last 3 months. please help.