You and your child are walking on a street and suddenly see a cat eating some meat.
At that moment, both you and your child know what the cat looks like (white fur, etc) and know what the meat looks like (red meat, etc).
I know that the more suitable expression should be "Look! a cat is eating some meat."
But I also wonder if the father and his child could say "Look! The cat is eating the meat." because at the moment of uttering both see the specific cat and meat and what they look like?