Your rates are too low! How many hours are they actually with you for before&after? In my area it's at least $25-30 a day based on roughly 4 hrs and a walk to dropoff and pickup from school.
Don't reduce your rates on holidays/PA days etc. The child is taking up a spot no matter what.
Fees should be based on normal weekdays of enrollment, regardless of attendance ie. if you normally come on Mondays, then all stats/family holidays on a Monday will be paid-period!
Take payments several weeks in advance of care. Personally, I'd find another client if she's giving you a tough time.