your child is underweight
to reduce mucus you should avoid direct milk intake for sometime,avoid curd
you should give her milk with tea leaves(little)
to increase weight please give her sweets, paneer ,Dal with ghee,butter ,dates ,puddings ,dry fruits .
you you are non veg you can give her non veg and eggs
when she is not having cold
,you can give her banana shake/mango shake twice daily.
till the time she cures her throat,avoid citrus and cold food.
i am sure ,it will help you.