I would let her have the opportunity to change her hours ( if you want to keep her). I would just say that the current hours are interfering with your families hours and that in order to have some sort of co-existence between work and home you will need to shorten your operating hours. Effective xxxxx(whatever the time frame is required for a change in your handbook) the new hours will be xxxx to xxxx. Let them know that you would like to continue caring for xxxx but you will understand if they have to make alternate arrangements.