If it were me I would either

A. Keep rates the same but cut back on the organic groceries or

B. keep rates the same and ask parents to supply lunch and either supply the snacks or ask parents to supply that too ( it would be interesting to see who sends organic meals now that it's on them)

I would explain the reason for the change being that you are no longer making a profit and in order to stay in business without raising the fee this is what you have to do. I'm willing to bet most parents will be fine with either decision so long as it doesn't affect their daily fee.

There are disadvantages to parents supplying the food but you have to decide if they are worse than not making a profit.