One response might be: "Kyle, I'm sorry your fish is dead, but it's really no big deal. He's just a fish. Death is part of life, and you need to learn that. You wipe those tears away, son, and go outside and play." Another might be: "That's OK, honey. You know, the fish was already old when you were born. We'll go to the store tomorrow and get you another one. Now put on that happy face, and go outside and play:'