I am wondering what the people who don't know that meat involves the death of some creature think of the Thanksgiving or other roasted poultry. Perhaps they missed the chart in the meat department that shows the cuts of beef and pork, so I'll give that a pass. The seafood department has whole fish; well, gutted and ready to prepare, but still looking whole. Lobsters are in the tank at many restaurants. (That one really bothers even me.)
I guess it just never hit them on a conscious level. 1 + 1 = 2.
Our grandson figured it out before he was three. He was eating a bone-in chiken leg, his dad reminded him that it was chicken. Grandson said, "Cluck, cluck?", son nods his head, and grandson after thinking a bit shrugged his shoulders and went on eating.