OK, maybe not as appropriate, but my DS's favorite joke a few years ago was:

How do you scare a bee?
You say "Boo, bee!"

And not a knock-knock, but an old family favorite:

Inch-me and Pinch-me went down to the river. Inch-me fell in, so who was left? (correct answer followed by gentle pinching)

Pete and Repeat went down to the river. Pete fell in, so who was left? [Repeat.] Pete and Repeat went down to the river. Pete fell in.....etc.