Of course, we all have our own way of doing things, and you may already have a great method for creating a snack schedule. But remember that Kindergarten is not only challenging for the children to begin with, it's also pretty daunting for the parents, too.
Not only does this create a financial strain for some families, it's a time gobbler, as well as just being pretty darn overwhelming! Consider splitting your schedule in single week blocks, or by allocating a certain day of the week for the next month to be one family's turn, and so on. So, every Monday in May will be A's turn, and every Tuesday in May will be B's turn.
This way it seems much more manageable for parents, both financially and otherwise, as well as giving them the opportunity to shine with an amazing snack once a week for a brief period. Give them a solid two-week block and you'll likely be faced with bags of goldfish and raisins -- great snacks, but boring if everyone does the same, just through pure fear.
Make sure to give your families plenty of notice, and then the next month, another five families get their turn.