The same thing happend to my daughter. She was sick for a couple of days, then got better, then got sick, then got better. And every time she got sick it was worse. I finally took her to the dr after she threw up twice in one day. He told me the same thing, that it was just a bug going around. I finally remembered what my mother told me that a dr. told her when she was sick with the flu, Change the tooth brush every day. I didn't change the tooth brush but what I did do was stick it upside down in a cup of listerine, the original. She got better in about a day and a half. I have since then kept all of our tooth brushes down in listerine. Think about it, it would be like using the same cup over and over and never cleaning it. If your sick, even if your body fights it off, when you use the cup or tooth brush again you get sick again. I wouldn't think that the kids BOTH being sick is all that weird. If they play with the same toys, accidently drink out of the same cup or if their toothbrushes are close enough together to touch, go to the same school, anything like that. I would also suggest to have her get into a habbit of spraying everything down with either lysol or listerine, the original, in a spray bottle every night before going to bed.