How To Properly Terminate A Family From Dayhome?

Alright, so I recently opened a dayhome, I've written up contracts, menu plans, and weekly schedules. My interviews went great, and all contracts were signed and agreed upon. I do enforce an illness policy where if your child has anything other then the common cold, they cannot attend , to protect the health of my family and other children in my care. On this particular boys' first week he brought croup through my house, and now my son is very ill.This boy missed his first day, and did not have a doctors appointment until Friday( which I was unaware of) and I had to call the parents over the weekend to find out what he had, because they didnt bother to call and let me know. I have already been pre-paid, and I am wondering how to handle the termination; how long do I give them, do they recieve their money back?( I only charge half price for the first month), should I send them home with a letter, or do it face to face. thanks for the help!