Some parents are always looking for the better deal, and it comes down to price with them. The others are right, you don't want them. Although I understand that some have tight budgets, if they are haggling with you over reasonable rates for your area.... forget about it!
Some might just be questioning, or curious. It can't hurt to ask, I suppose. If anyone questioned what I charge (and I know it's on par, or on the lower end, for my area); I would simply and matter of factly explain that my rates are set and I don't negotiate. If I was you, I would absolutely remind them of your whole food/ organic policy, etc.
They either agree to my pricing, or not. Their choice.