The timing was unfortunate, but work trumps parties/get togethers (except possibly your wedding or one for a close relative). Refuse to take care of a child sick with a "bug" and most families would have to reevaluate why they have in home care.

Occupational hazard.

Sorry, no sympathy here. Try taking care of four vomiting kids at the same time, then get back to me.

-Nanny who has dealt with much, much worse