I would be honest if I were you. A good reputation is important because sometimes it's a small world and you want all your clients to say good things about you. Of if you decide to wait and see if you can fill their space with a full time client make sure you give the current family lots and lots of notice and explain that you had to make an important business decision for your daycare and your family finances. It's always best to have great relationships with clients even when they are leaving the daycare.