Daycare and being fair with what I charge?

I have been watching a 5 month old girl in my home. The parents and I agreed on a flat rate per month with me watching her Mon-Thurs. I have recently been watching her Fridays at no extra charge. My husband and I will be going to Vegas next week and I will miss watching her for 3 days. The dad asked me today if he had to pay me for those days. I personally feel that they should. I would for someone that is bending over backwards for me. I have picked her up and dropped her off before. My question is: Should they pay me? Thanks :)