Just do what me and a mate did with his little sister....
"No...the fish isnt dead...he had a cough so we took him to the doctor..."
Meanwhile the cat was licking his paws after yet another easy meal...
Worked untill she was 5 atleast!
We originally had two fish, when one died We fed it to our cat, but he didnt seem very interested, although he does get fed fairly well. When my daughter (3 yrs old) saw it lying on the bit of newspaper she said it must have died by jumping out of the fish tank .