Foods to boost Immune system in Toddlers and Pump Suggestuins

I posted this question earlier so this may be repiticios.
My 2 year old grand daughter was diagnosed last March. She just seems to get everything going around and is sick a lot. She has a 4 year old sister and is in day care. Does anyone have suggestions for foods to boost her immune system? She won’t take vitamins and of course is a picky eater.
Also her doctor has just recommended a pump. She is not very stable and has a lot of scar tissue. They are just beginning that process, so again, any suggestions for pumps for a 2 year old would be appreciated.
Thank You!

My wife and I are avid users of essential oils. We use lots of thieves oil and eucalyptus oil. Our 3 year old was diagnosed two weeks ago. I have a feeling that we will be using them more now.