I would be angry, Bridget. It's one thing to think it is a cough, but if she had any inkling at all it was something more serious than she should not have brought her kid to your daycare. I would definitely take your kids in to the dr. after hearing that, or at least call them to let them know so they can tell you if they think they should be brought in. You don't want to mess with pertussis. Untreated it can become dangerous.
"Any idiot can face a crisis; it is this day-to-day living that wears you out." -Anton Chekhov