You could also advertise now with the opening dates as July and September( if those are the correct months) and see if you can find children to fill for then. If June rolls around and you have had no interest, than you could consider the option of terming and accepting the new mom.

Have you spoke to the current parents and asked if they will for sure be staying until July and September or asked if they have alternate care if you need to fill the spots before then? If new mom can find alternate care until at least the first spot opens, that would be my preferred plan.