As mentioned you haven't stated the disability so this may not apply. But my son is a very picky eater and I dreaded day care meals an hated seeing the board ticked 'some' which basically meant he ate none.
Over a few months though since he has learnt that there is nothing else... He has learnt to eat it... Now I see 'all' ticked and the day care staff are wide eyed with what he ate. I now have the opposite effect as my son is a really picky eater for dinner and knows I will offer fruit and yoghurt if he doesn't eat it. He actually hands it back to me and says 'yoghurt please'
It's one meal and he won't starve to death not eating it. Personally I didn't expect his daycare to offer an alternative and I agree all the kids may want what the alternative is, plus I don't think it sends the right message to the kids.
If your really not comfortable then maybe it is worth looking into daycare options where you bring your own food...