Which day is she deducting?

I get that she deducted the Thursday where you closed....

But is she deducting Wednesday where she took her son to the clinic or the Friday where the dad is staying home with the DCK??

Either way....she should pay. It was her choice not to bring the child.

I would ask her why she is short and see what her justification is. Then I would remind her about your policy that she has to pay when you are open and that she is paying for the spot - not by attendance.