You can't boost the immune system with a single vitamin (such as C, or anything else) or herbal treatments (such as echinacea) - the body needs a balanced supplement with many vitamins & minerals. Every vitamin, trace element, phytonutrient, etc., works in the body in conjunction with every other one - so the partnership and balance are critical! You can't find that in the health food stores but there is a company that manufactures something here in the US. It's used all over the world including in feeding stations with really compromised kids. The company connects you with a trained nutritional consultant to help you. If you want to go that route, I'll help you.
I'm glad the other poster has had good luck with Flintstones, but I wouldn't give my kid something that says "keep out of reach of children" - I'd use something with NO warning labels and that I'm sure is not outsourced and manufactured somewhere else.
Trying to mix & match various ingredients and playing "kitchen chemist" yourself is really impossible to do well. Most people are just wasting their money buying commercial vitamins. 80% of the population supplements, but would you say 80% are healthy without illness or immune issues? Nope!
I've gotten rid of a huge number of viral issues (colds, flu, chronic bronchitis) by doing comprehensive supplementation. Haven't been sick in 5 years, my doctor is thrilled. My husband has had quite a number of warts and other skin issues disappear, and he's never been sick either.