Hi Adara
A simple, tried and researched method to improve immunity in Kids and Adults:
Increase the intake of Garlic.
I used to get frequent flus while in college, whenever I go to mortuaries, then one of my seniors advised the below and I tried it and unbelievably it worked for me.
Take 2,3 pods of garlic and boil in milk until it becoms soft then crush the garlic and drink with the milk. This I did for few weeks and After that the frequency of falling sick reduced dramatically.
I follow this to my kid too. But he wont drink the garlic with milk ao I add garlic liberally in all foods making sure that his intake is atleast 4 or 5 pieces of garlic per day. Touchwood he is immune enough to common colds/flu for a primary school kid.
Also I make sure the diet has 2 servings ( a fullcup= 1 serving) fruits, 3 servings of veggies everyday apart from milk/cheese and egg everyday!
I give leeway for fast foods too once a week one meal can be his choice of junkie like Mac, Burger King, KFC or Pizza Hut.(tat way I can curb his grumblings of having to eat veggies everyday

)
Last week I read in the news paper that there has a been a research result published (in the USA) saying that the intake of Garlic is found to have improved immunity in human beings.
(Personally I am not in favour of giving health supplements pills(over the counter drugs) to kids since I am scared of Oversupplying than what the body actually needs and long term oversupply could be hazardous to health) But we have to check with our paeds if the kids needs any supplements and ask them to prescribe.
My 2 Cents!