|I have a greater moral objection to criticizing the food laid out in front of me
What if it was people? Like, a human head?
Because moral vegetarianism is for many based on the belief that all animals deserve the same reverence we accord people, at least with respect to whether or not they may be killed for food.
Do you believe it's wrong for Jews and Muslims to politely decline pork or other non-kosher or non-halal foods? ("Thank you, I do appreciate it, but I really can't. The salad looks fantastic.")
I think that you are really not taking the vegetarian ethic very seriously. You will see that I argue very much for grandparent's rights, even to feed candy, watch TV, etc. but I also think that asking someone to abandon their parent's philosophical beliefs or religion is just not right.
And I do think that is probably where the grandparents here are not getting the point either. They simply do not and can not understand the moral objection to eating meat, that it's not like, "Oh, don't hurt a cow!" when life is full of pain and vegetarians know that.
It's something much different, an entirely different way of looking at animal life.