Ettiqutte help!!!

One of my coworkers invited me and my children to a party at the zoo. We were given two vouchers to get into the zoo for free, anyway, my children this morning have the bad case of the sniffles and I am in a bind on what should be appropriate way to handle this. I don't want to take them to the zoo, not only because they'll probably eat alot of sugar which would make them worse or even be out in the cold, but also the possiblity of getting other little children sick, but on the other side of it, I'd hate to call the woman up and tell her that we couldn't make it, even though she went out of her way to spend money on vouchers and make t-shirts.

So...which would be better, or which is less rude, since it's going to be rude either way I go.

ugh. : (