Do not tell her you cooked the fish, she is too young. I did a similar switch one time and my kid figured it out, but he was about 8. She is one smart kid. What good would it do for her to know they are dead and then you get into a discussion of death. Stick to the they are sick explanation until she forgets about it, but with that smart a kid you are not going to be able to do that trick again. She is already on to you.
