I don't think you were mean, but maybe a little bit irresponsible. When we have a virus, we don't let anyone in the house- including our housekeeper who is amazingly helpful. If we're sick enough for me to stay home, then I figure it out. If she gets sick, she brings it back to her children and her family and then she's no help to me when we're back on our feet.
What should you do? The "right thing" to do would have been to call her ahead of time and tell her not to come in. I'm guessing that you don't have a written contract with her, so you would not have paid for the days you were home caring for your children. Instead, you elected to expose her and now you've paid her for her work time and you should pay her for her sick time... not her hospital bills, but the time she would have been working had your family not infected her.
Be prepared for her to be quite unhappy when she comes back, as this is going to cost her a lot of money. If she needs to work "off the books" to make cash did you really think she would leave on the those days?
You could offer to pay a portion of the bills, but I wouldn't suggest it. That opens up a flood gate that you can't afford to deal with. I would offer her paid time off until she is feeling better and go from there. In the future, if your family has something contageous, deal with it yourself and don't invite anyone into your home!