Personally I think she is pushing her luck more and more because you have let her. If the child has been there more than a year and Mom has been taking lots of "free" days off from the start why would she start paying now??? You're contract is useless in this regard as you have let her do whatever she wants. If you don't follow your own rules why would she?

Cut her loose and start with a new family. Make your rules clear right from the start, no exceptions.