Most of my "mysteries" like this end up either being a fish that jumped and was eaten by the cat or dog, or I end up finding some sort of skeletal remains when doing a good cleaning. Some fish, I honestly never see, but thos have been small fish in a tank of large ones. If the fish are the same size, I doubt it.. the remains may have slipped under a rock, or he's hiding really good.. I don't think your kids ate him.
Then again, he good have 'poofed' to a parallel universe.. hmmmm...