I don't have any kids in daycare, but I can say that feeding my kids all-organic, no-trans-fats is a deciding factor in a lot of decisions, including previously my decision not to take 'advantage' of the free/reduced price lunch program, which I'm certain we qualify for ... but we are at a charter school, and I recently found out they have worked out a way to make that program work with food from New Seasons, so I'm TOTALLY applying this year! I may donate the money I save on lunches to the school at Annual Appeal time ;).
I think $5/week is a fabulously small amount to pay to take one more thing off the list of 'things a mom can feel guilty about not providing her kid(s)'--and we all know we keep those lists--or rather, they seem to keep themselves, in our heads ;) !!
You might present it as "$7/week," and then either let them 'argue' you down to $5, if you think you have bargainers in your parent-crowd ... or figure that every three that agree pay for one that doesn't, or offer refunds at the end of the month(/credits for the next month) if you brought it in 'under budget' ... . Or announce it as part of your regular price increases in January or June or whatever, without too much explanation.
One thing: I shop at New Seasons and not at Whole Foods, and I love love love it, but New Seasons doesn't have the no-trans-fats policy that Whole Foods (and I think Wild Oats?) does, so while you are building your online shopping 'favorites' list, you would need to do some initial label-reading to make certain you don't choose any of the few items they carry that don't meet your plan. (They consciously have a 'well, everyone has a vice or two' sort of selection ;). )