First I would try to find a different reason to tell the families about why you are letting them go instead of others to soften the blow a bit - think about age, sleep needs, independence etc.

I would think the summer will be long enough but it will make them panic and especially if they are taking July holidays. I might consider making the changes happen in June and enjoying a more carefree summer with your son and then come September you will be in the right routine. Which means you should be giving an April/May notice for June or a June 1 notice for September so they too have time to look for care before holidays. But assume they will leave sometime over the summer as soon as the other caregiver has a space and there really isn't anything you can do to stop that unless you want to only give them the two week notice in your contract.