My BABIES were exposed to children with CROUP a couple of days ago! HELP!?

I have a 3 year old boy and a 9 month old boy. The other day we went to our friend's house and their little girl came down with Croup while we were there(her brother had it the week before). I tried to keep my baby away, but he crawled over and put her pacifier in his mouth. Is there anything I can do to build their immune systems enough to ward it off? It's been 2 days since exposure. I have baby vitamin c tablets that melt in you mouth and Chance drops that I've been giving them. I started just today after I bought them.