I think that it doesn't hurt to get the parents' opinions on this. You could do up a quick questionnaire that asks if they like packing their own food (and why), whether they would be open to the provider supplying food (and why), what types of food they would like to see, would they be willing to pay extra and how much (give them a few options, don't leave that open to their interpretation), and so on....
Another option is to begin implementing a food guideline in your home. Send home a list of acceptable foods and unacceptable foods. Tell parents why this is being brought in (for the health of the children, and in fairness to everyone, etc). Let them know that should foods on the "no" list show up, you will be providing a suitable alternative, possibly at an added cost. Or you can give them the option; either you provide meals (with an extra fee attached) or they follow the guideline.