Seems so many kids with long term cough. Make sure you know what exactly is the cause of the cough. Ask 100 parents will have 100 different methods of curing the cough.
Some cough is due to milk/protein allergy, so a parent will tell you that change of milk cured it.
Some cough is due to asthma, so a parent will tell you some herbs with plant alkaloids, say ephedra, cured it.
Some cough is due to food allergy, so a parent will tell you diet avoidance, say peanuts or seafood, cured it.
Try 湯水 is fine, I take it everyday. But knowing why the kids keep coughing is also important.