I think that having your husband or a grandma as your own back up is a great idea. When I have my daycare set up in it's new space, that is my plan too. I feel the same, that if I ask parents to keep their kids home when they are vomiting, etc then I should do the same with my own.
On that note, if my own kids are sick, I give the parents the option of bringing their kids. Most of the time, they say no thanks and keep them home or make other arrangements. When daycare kids are sick, the arrangements are up to the parents. I state up front that I do not provide for back up care and that it is the responsibility of the parents to have a plan B. If I interviewed someone and they had no family, no friends, no one who could help them in a situation like this; I probably would not take them on. Sounds harsh, but in the long run I don't want to leave them stuck in an emergency. I don't close often, but it has happened.