Oh, I'm so sorry! Like mamabear said, how they deal with finding back up care is not your concern, so don't even think about it.
In the future you might consider insisting that parents have an emergency back-up plan- I do, and I never feel bad about taking a day off for doctor's appointments, illness, etc.
It's right in the contract that they all sign - "please understand that you must have back up care in case of emergency or illness on my part. It does happen, and I suggest that every family be prepared"