Just don't do it. You slap a diaper on that kid whenever YOU see fit. I will not allow dcparents to tell me when their child will or will not be in a diaper in MY care. I am the one who has to clean up the mess. I am a reasonable person who has a lot of experience with children and I know when a kid is ready or not. Besides, it's unsanitary for the rest of the kids to be exposed to some other kid's pee and poop.
This happens a lot, BTW....parents always want the provider to potty train when the kid is about to leave care. I refuse to even help if that is the case.