This is why most if not all providers charge for the spot. Not the days used. You just lost a whole week's income from this one client. Whose to say that in future someone won't abuse this policy eg. saying they are sick for a few days a week in order to avoid payment and then having grandma or another family member watch their child.

I would amend your policy ASAP. Tell your client that you cannot afford to do this in future, or, give a 5 day sick leave per calendar year if you want to continue this arrangement. Personally I would put a stop to it.