Wow I would be freaking out too. I know had it been me I would have told them right up front when I got the job that I wouldn't watch their kids if they were throwing up. I know that's kind of cruel but with my phobia and with not wanting to get sick that's how it would have to be. Since you are there and there is nothing you can do I think you will just have to take some deep breaths and hope she doesn't get sick all over the place (that would be my fear). Make sure she has a bucket by her and let her know to make sure she aims for it or makes it to the bathroom if she is going to be sick. Show her some compassion because she is sick and keep your hands clean and dont' touch your face!! That's about all you can do and hope for the best. And pray this bug doesn't get you.