yes I have heard of USANA & from what I understand the vitamins have higher doses so that has to be good for you, some more common ones are Elivet - which is a multivitamin for pregnant & breastfeeding women so if you are going to take that you should probably also look at echinasea (sp), vit c, garlic, all things to boost your immune system. You could try some soup that has heaps of garlic & celeray in it for their goodness as well.
Unfortunately working with kids you are going to be at higher risk of colds etc... you can get instant sanistiser cream stuff that you put on your hands to kill the germs, that could help to prevent future colds. Goodluck with it