A Certain Strain of Funny

I have tried to keep my children clean of "potty humor." But I am up against the Stevens's on one side and a few "Ghost Poopie" afficianado's on the other, as well as the public school system and all the "jokes" my son brings home.

So when Lemur tells a joke, it goes a little something like this:
"How did the cow learn to fly?"
"How?"
"Because a squirrel pooped on a diaper on his head and fell!"

Or

"Knock Knock."
"Who's there?"
"Diaper."
"Diaper who?"
"Diaper full of poop that came from a donkey and fell on your underwear face!"

I have tried to discourage this strain of humor. I have even tried to teach him the proper forms of higher joke telling, putting emphasis on wit and pun rather than a disgusting and improbable punch line. My example was,
"Knock Knock."
"Who's there?"
"Banana"
"Banana who?"
Knock Knock."
"Who's there?"
"Banana"
"Banana who?"
Knock Knock."
"Who's there?"
"Banana"
"Banana who?"
Knock Knock."
"Who's there?"
"Orange."
"Orange who?"
"Orange you glad I didn't say Banana?"

The humor was lost on my poor son. And apparently it was also lost on my daughter who joined the fun by saying: "Knock Knock."
Leemur answers, "Who's there?"
"Banana."
"Banana who?"
"Banana Poop!"

They'll grow out of this, right?