Disclaimer: I am not a Doctor or qualified in any form of Medicine, so please do not rely on my advise, and always consult a doctor if concerned.
My son despite being away from the plague factory that is his play group, has a stinking summer cold, as well as mandatory hand washing every 5 minutes I am dosing everyone in the family twice a day with Echinacea and so far that appears to be working!
I use a remedy from A Vogel for children, and their website describes it thus Echinacea works by balancing the immune system so that it works efficiently. This means that you are less likely to catch colds etc, and may be less prone to allergies. The plant itself is rather pretty and I think I saw some growing in a garden recently. It actually comes from North America where it was an Native American remedy. I find the taste quite nice especially in tonic.
But please follow the guidelines on the bottle and or your doctor