N.B.
Hey there, J..
I know it can be hard when you have a great relationship (and baby-sitting situation) to bring up sensitive issues, but you do need to talk with your neighbor. You could bring it up in a non-confrontational manner to find out what she does for prevention or treatment. She might need some help in that area... especially if the family always has colds... and may be thankful of some suggestions or advice. I would approach it by saying something like "I went on-line to ask other moms what they do about kids getting colds all the time and got some suggestions like..." (it's a thought anyway). ;o)
In my opinion, the best way to prevent spreading colds/viruses is to wash your hands well after being in contact with the snot or things that have gone in mouths, etc. so I think it would be a good time to teach the other children the importance of cleanliness if they don't do that regularly already. Using hand sanitizer like Germ-x helps as well (use only things with alcohol base, NOT Triclosan).
Another thing to remember is that pushing clear fluids, like juice and water, is important when the kids have infections so their little bodies can flush it out of their systems much more quickly.
When my little boy gets sinus infections or colds, it invariably leads to chest congestion and coughing and then ear infections (hence time out of daycare for him and work for me) so I completely understand where you're at. I've found that making sure he is getting his daily vitamins (I haven't always been the best at that) has made a HUGE difference for his immunity... and letting him have as much vitamin C as he wants when he's got a cold seems to help as well. I buy a roll of those Daily C tablets and let him have all the "candy". He loves it because he doesn't think it's medicine and I know he can have them all because he'll just excrete what he doesn't need. Hope this helps... good luck! ~Nicki