What are the natural sources to increase immunity in kids against cold and cough??

On top of all that have been mentioned, I would recommend probiotics to help boost immune system (though not directly related to cough and flu). Our digestive tract makes up 70% of our immune system. The main strains in our bodies are Lactobacillus acidophilus and Bifidobacterium. They act to prevent malabsorption and gut permeability then in turn increase immune function. Yogurt and cultured milk drinks are good options. Best to choose the low sugar (or no added sugar) options when available.