Staff "refusing" to work a schedule

What we have done in the past is split shift so each can have some of the day off. If its important for a single mom to be there christmas morning then maybe the other one would work that part so it would work out for the best. Maybe she could work a shorter shift or come in later and stay later. There is always room for compromise. But the refusing to work needs to be addressed because once one is giving a little extra room and allowed to refuse they will think they can do it for every situation.