Give them a better chance to fight off some of the stuff with a high quality vitamin (flintstones just don't cut it)
I give my daughter Might-a-mins. It is a powder that you dissolve in water and they drink - it tastes great, kind of like orange soda. When it is mixed properly it becomes isotonic (which means that it will spend almost no time in their stomach, it will go straight to their small intestine and get absorbed almost completely) They have many key vitamins but also a great probiotic. You can only get it thru www.marketamerica.com
My daughter used to get sick all the time until I started giving her this and now she only occassionally gets a cold plus - when she does get sick she's over it in about 2 days instead of a week.
Also - try to teach them about how they are washing their hands . . . well, the 3 yr old at least :) Kids are SO BAD at washing their hands and it is one of the best preventatives to getting sick.