I had a contact and was still walked all over this first year too!
So the dad is in retail, I get it. They need make their lives more structured (for their sake and yours!) and he needs to tell his boss the two weekdays he's not available to work and the kids are with you the other three. End of story. They pay for it or they lose it. My Dayhome families know that with any less than one month notice I'm probably not able to accommodate requests for changes/ day switches. Even with notice sometimes it's just not possible. They can't expect full time availability without paying full time rates (I've used that line with parents).
It's hard to tell parents that they need to get their lives under control and get organized - I'm sure you can find a nice way to say it. Kids (and everyone) thrives with structure. If they don't understand your need for structure, start looking for new clients. I highly reccommend buying the online video course how to run your home daycare - by Jana - I forgot her last name. I wished I had gotten it earlier. Full of brilliant tips to rock the Dayhome thing.
I'm rewriting my contract for the next clients now that I'm done being walked on. Happy note: not everyone walked all over me - I'd go to the moon for the respectful parents - I hope they stay forever!