"Daddy, Daddy, you know you're always saying pigs might fly? They actually did! I heard it on the news." "Huh, really?" "Yeah, they keep saying swine flew."
Is it likely that a child might know the meaning of the word 'swine', but not understand ''flu'?
Or is the joke that the child thinks it is funny (it's very much the kind of thing that children find hilarious; see also 'Who's there?' 'Doctor', etc) when it actually isn't?