If you give parents as much notice as possible so they can make alternate arrangements for care or call in sick themselves that's the best you can do. Emergencies happen and most parents are fully aware of that fact.

If I close for an emergency I don't charge my clients, but I did write 3 paid personal days into my contract in case I'm too sick to care for the children. You know, it's usually the daycare children who bring the germs to our homes, so don't be afraid to remind your clients of that.