It's APPALLING that they're feeding that stuff to the children in the first place and truly horrendous that they're trying to dissuade you from making better choices for your own child. I'd go to the waaaay top. I'd send a letter to the First Lady and cc the accrediting agency of the preschool. No school should be serving that "food," and no school should be penalized when a parent wants to send in a healthier alternative. I would think that with all the work the First Lady is trying to do to combat childhood obesity she would be very interested in a top rated preschool serving junk and being penalized in its ratings when a parent protests.
In our neck of the woods that preschool would be out of business very quickly. Our preschool gets flak for serving fruit juice three times a week (many of the parents opt out and choose water, which they are absolutely allowed to do) and giving the kids occasional ice pops in the summer for extra hydration.
I sorry the school is interfering with your better choices. Preschool is really too young for the school to be undermining the parents' choices. It's bad at any age, but it's really unconscionable in preschool.