Consult your child with Ayurveda doctor, having cough for long is not good.
You can apply sesame oil over chest & if possible use hot water bag .It will help to liquification & expulsion of cough.
Next Steps
Consultation with Ayurveda doctor
Health Tips
Avoid-Rice,Milk products,Sweets,cold water, icecream
Use - Warm water for drinking,Warm clothes, Give Honey +Yashtimadhu