Is it just me, or does slipping a letter in the diaper bag for them to discover when they get home kind of passive. What if they have issue with it? Do you wait for them to bring it up?

It just feels weird to me that they would come and pick up their child, you would smile and say have a good night and then have them find out at home that you have an issue with them? I think I would be irritated if that happened to me, especially from a home daycare provider that only has a handful of kids. Maybe it would be a different situation if it were a daycare center where there maybe be varying staff interacting with parents on a daily basis.

Maybe giving the letter along with a face to face conversation, so they have the info to take home with them.

But, for sure you can set the payment dates and it is up to the parent to make it work on their end financially. It is their responsibility, not yours.