Wow... that's fantastic! I would love a parent to bring food. I remember when my kids were in daycare I started packing them a lunch when they were close to kindergarten age to get them used to opening and closing things on their own.
I would just politly remind the parent that you are happy to serve them lunch. But they need to commit one way or another.... if you're like me you probably plan your groceries based on how many children your feeding.