They have failed you, not the other way around. I wouldn't renew if I felt confident that I could fill the spot.
I have a very hard time working with families who don't seem to care about their children's care.
If you would like to keep them, I would say "these are my requirements:" and specifically list what you need from them, such as a set schedule, payment in advance, etc. If they don't agree, or break the deal later.... get rid of them.