N.,
I feel for you! Unfortunately, I think your kids are simply at the prime age for catching everything. I thought the same thing when my first child was in daycare. It seemed like she caught any and every virus! My pediatrician assured me that it's all just part of building up their immune systems.
When I asked her if maybe I should stay-at-home since my child seemed to be sick all the time anyway, she said no. That either they get it at daycare now, or when they go to school at 4-5-6.
I too have my kids wash hands all the time, use hand sanitizers, etc. I do think it helps to have them wash their hands in the car, as soon as they get in their car seats, before their snacks. I think this has significantly helped cut down on bringing the germs home.
Lately when one kid gets sick I go crazy cleaning and trying to keep their germs confined - washing clothes, sheets, dishes, etc. immediately after use. I also found that when one kid is sick, I put a clothespin on that towel so NO ONE else uses it, even to dry hands. We "blow" kisses to each other. This has really helped keep everyone else well, even when one person get sick. (OK I confess, I did break down and buy the Clorox wipes the last bout of perforate ear drum and strep. I was determined to not spread it around.)
The only other thing I can suggest is trying to teach your 5 year old good hygine manners at school. As much as you keep him and your house clean, if he's getting coughed on or sharing toys with sick kids at day care/school there's not much you can do. I have worked hard to teach my older child that the sick kids at school can make you sick. So be a good friend and offer them tissues or to go wash your hands with them at school. Please don't share toys, food, etc. with them. After this last round of illnesses, I think she finally "got it" and understands that not keeping herself "clean" at school can affect her later.
Best of luck. I know it's hard and frustrating dealing will all the illnesses...especially when they spread in the family. Just know that everyone else is in the same boat.
BTW- my pediatrician said the multivitamins aren't going to fight off a virus or bacteria. It's just their immature immune systems getting overloaded. Just do your best to keep them home if they are sick, so you don't spread it around to everyone else. (I know, easier said than done. I feel like all winter I never walked into the school without seeing 1/2 the kids with runny noses or coughing!)
Best of luck!
Sara