are you a nanny? they can set your hours to what they want, unless it says they can t in your contract. if you don t like your job, get another one. that s what i did =] i also work in childcare.
Was the prearranged schedule in writing? If yes, obviously the drastic change in hours is not legitimate. However, since this company appears a mom and pop operation, you are at the mercy of the owners. If you have a good relationship with the boss, try to have a talk with her, and explain your predicament. If she does not seem willing to reach a reasonable compromise, you may have no choice except to put up or leave.
Did you find the job through an agency? It may be worth talking to them to say that the hours are not what their client asked for when they asked the agency to find them someone.