Like a lot of this stuff if the kids are given a good enough grounding at home it shouldn't have much of an impact. And like all things in moderation the occasional junk meal isn't going to harm kids., any more than the occasional drink isn't going to harm an adult. Its excess thats the problem.
Stopping the kids from having a mac Ds is more than likely to make them want them more, in the big scheme of things an occasional MacDs amongst a couple of months worth of good food is going to make no difference to an active healthy child. Whether the benefits of the school sponsership outweigh the negatives I will leave to others who have the figures in front of them to debate.