To renew or not to renew

Here's my story. I've had a 3.5 year old since September of this year. The first contract was for 3 months, as they always are in the beginning. I renewed for another 2 months at the last minute. I've been asking for post-dated cheques since day 1 and each month I have to remind them over and over to pay me. I said I would not renew until I was paid, and had the post-dated cheques, which they brought the day of the renewal.
Since then, their schedule has dramatically changed. she currently goes to preschool 5 morning/week, but they are in the process of changing that. I have to ask each morning if she's supposed to go or not. this morning the dad says. "I don't mind. As you wish" Well, that means no arrangements have been made on their part with the school for the bus...?? It's not MY decision. this conversation took place after he raced up to where I was waiting for my son's bus and sqealed his tires he was going so fast!
It's so hard to describe it all, it's all just weird:
late payments
loooong hours
me having to communicate with her school about transportation...??

So, I'm debating if I just let them go at the end of the 2 months. I always feel like I've failed somehow when it doesn't work out....

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.