This can be very tricky obviously! When I chose my clients, I chose people that I could imagine being friends with. Not so that we would be friends but because that made sure I only had clients that I would enjoy having! Not once did I socialize outside of the daycare. Now that I have moved and no longer provide care for the children we get together frequently for bbq's etc. I would simply say that you were unable to attend an event that they invite you to, you don't have to explain about keeping things professional. Be friendly not friends.