As hard as it is the sooner the better would be the best time to let the family know so there is some time for you both to grieve so to speak and get used to the idea of the changes.

If this is the last client, do you know someone else that does daycare that has a space that you would trust if you were the parent looking for care and could recommend? That might make it easier on both of you.

You really should have given them a month's notice to be fair since you know they are going to have a hard time finding someone else.

Another thing to consider is how does this child fit in age wise with your own kids? I found having the daycare kids was a great diversion for my own kids and the bit of extra money helped to offset the pizza ordered instead of cooking. That would allow you to give them notice for say Thanksgiving or later.

Another option is to find them a friend with a space and say this person will take them while you are on break due to the fact you don't have an assistant now and then after a month just let them know you won't be reopening. Hopefully they just stay where they are but at least they will have care till they make new permanent arrangements.

In the grand scheme of things it isn't our responsibility to find them care. Just note what your contract says about notice as you are likely cutting it pretty close for closing on them ie minimum two weeks notice and we usually give our best clients a sooner heads up.