How will parent check if you are serving organic food?
LIE.
Plus, if you serve party food to children, how the hell do you tell them they can't have the nice stuff, just because 'Mummy has issues'. You can easily do halal for everyone. If you have a child with allergies you have to police them / be careful with the food - and that's fair enough. But you can't police ALL the kids and you shouldn't be expected to.
Its not a dietary requirement. Its being pretentious and over privileged.
There is no such thing as 'bad food' only food which you should eat in moderation.
As for 'eggs are not vegetarian', just ask them what is the difference between their vegetarianism and veganism and 'vegetarians' who eat fish. Then say you only do eggestarian. With a straight face.
I'd see it as a service to the child to give them crap once in a while. There's nothing wrong with the odd Maccas. In fact, next time, just do yourself a favour and say you are going to McDonalds.
Honestly, I'd cater for allergies and religious beliefs quite happily. I'd be fine with doing some vegetarian options for everyone just for a healthy alternative if they want to choose it.
But if you are going down the vegan or organic route, I would tell them to sling their hook and provide a packed lunch and have done with it. You can not accommodate everyone, and its hard enough to provide for genuine need rather than people who really do need a reality check. Not everyone can AFFORD to get in caterers, never mind provide for their whims.
Are they really going to stop their kid going on a school trip or scout etc camp because they don't do organic food. Do you think a school / scouts would accommodate a request for organic food?! Would they hell! Plus I can guarantee that given the option without overbearing parent watching, lots of these kids would go straight for the processed crap, precisely because they are not allowed it at home. Far better to teach the kids to make sensible food choices themselves from a young age.
As for being thought of as being rude, I wouldn't have a problem, because I know who the one being rude and it sure as hell isn't you.
TBH, if I got a request like that, I would take great pleasure in deliberate feeding child Asda Savers. No. In fact, I'd see it as a civic duty.
Organic fruitloops you have been warned. I have a nut intolerance.