My little’un has started getting into Knock-Knock jokes, but still doesn’t really know any and I can’t remember any simple ones.
Anyone?
Knock Knock
Who’s There?
Dwayne.
Dwayne Who?
Dwayne the bathtub, I’m Dwowning!
Sure. You start…
Okay, more seriously:
Little Old Lady.
(I didn’t know you could yodel)
Knockl know!
Who’s there?
Gestapo!
Gestapo who?
Ve vill ask ze kvestions!
Similarly:
Knock knock.
Who’s there?
Boo.
Boo who?
Stop crying!
A classic that the little 'un won’t get: “Anna Maria Alberghetti in a taxi, honey…”
Who’s there?
Sam and Janet
Sam and Janet who?
(singing) Sam and Janet evening…
The horrific classic:
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 saw banana?
My favorite:
PERSON 1: Knock knock.
PERSON 2: Who’s there?
PERSON 1: Interrupting cow.
PERSON 2: Interrupt -
PERSON 1: Moooooooooooooooooooooo!
Or the variant:
PERSON 1: Knock knock.
PERSON 2: Who’s there?
PERSON 1: Interrupting starfish.
PERSON 2: Interrupt -
(Person one puts their open hand on Person two’s face)
Knock knock.
Who’s there?
Amos.
Amos who?
A mosquito.
Knock knock.
Who’s there?
Yeti.
Yeti who?
Yeti 'nother mosquito.
Knock knock.
Who’s there?
Atha.
Atha who?
A third mosquito.
Knock knock.
Who’s there?
Airs.
Airs who?
Airs a lot of mosquitoes round here.
Knock knock.
Who’s there?
Les.
Les who?
Les kill 'em all.
Knock knock.
Who’s there?
Isabel.
Isabel who?
Is a bell necessary on a bicycle.
Knock knock.
Who’s there?
It’s me, your bell’s broke.
Knock Knock
Who’s There?
Eskimo christians.
Eskimo christians who?
Eskimo christians, I’ll tell you no lies.
Poysin: Do you want to hear a knock knock joke?
Kid: Yeah, please
P: Okay, you start
K: Knock knock
P: Who’s there?
K: 
Have the person raise their index finger and spin it around in a circle. Then:
Knock, knock.
Who’s there?
Woo.
Woo who!
End of joke.
Sometimes you can add on a “Gee, it’s not that good of a joke.”
For a somewhat older child…
Knock Knock!
Who’s there?
Urinary Tract!
Urinary Tract who?
Urinary tractive woman, you know that?
The interrupting cow one cracks up my 7 year old every time. So I second that one.
For the really little 'uns:
Knock knock
who’s there?
atch
atch who?
I’m sorry, I didn’t know you had a cold
knock, knock
who’s there?
who
who who?
Are you an owl?
Knock knock
Who’s there?
Olive
Olive who?
Olive you.
Knock knock
Who’s there?
Mayonnaise.
Mayonnaise who?
(sing) Mayonnaise have seen the glory of the coming of the Lord…
Okay, the “interrupting starfish” just killed with my 17-year-old and her dad. Thanks, storyteller0910!
My favorite series (keep in mind I produced a 17-year-old who found the above joke absolutely hilarious):
Knock knock
Who’s there?
Ether
Ether who?
Ether Bunny
Knock knock
Who’s there?
Cargo
Cargo beep-beep, run over Ether Bunny
Knock knock
Who’s there?
Boo
Boo who?
Don’t cry; Ether Bunny be back next year!
Knock knock
Who’s there?
Control Freak. First person continues “Okay, now you say ‘Control Freak’ who?”
Knock knock.
Who’s there?
A stupid burglar.
Using knock-knock notation…
K*(Shelby)
Shelby coming round the mountain when she comes.
K*(Huu)
Funny, I didn’t think you were an owl.
My dear child: Knock, knock
Me: Who’s there?
Mdc: ellyphamp (elephant)
Me: elephant who?
Mdc: Your nose is big and you have a stinky butt!.. Daddy, why aren’t you laughing?
I need to have a talk with their teacher.
I liked the one at the very end of Wil Smith’s movie, Pursuit of Happyness.
Knock knock.
Who’s there?
Nobody.
Nobody who?
(First person remains silent, hopefully leading other person to repeat “Nobody who?” until they get it.)