I think you almost HAVE to be honest and say that you are staying open but are choosing to concentrate on your social work and your son. It would be a big no no to lead them on and make them think you are shutting down and then they see you out and about with the daycare or hear you are still open through the grapevine.
Sounds like you have the perfect scenario to say that you are letting them go because you have promised the spot to your original families and that you find the PT schedules too hectic.
A lot of PT families get dropped for FT families so it is a good reason.