One of the many reasons these are not funny is that although many Japanese words sound similiar, bringing them together in a sentence provides a play of sounds but not of meanings juxtaposed enough for there to be a shift of expectation or a groan of recognition. But Japanese children find some of these to be amusing enough to torment adults with them. The key to their amusement is is a sing-song repetition.

Mushi wa mushi suru. = Ignore the bug(s).

Ika wa ikaga? = How about eating squid/cuttlefish?

Iruka wa iruka? = Is there a dolphin?

Futon ga futtonda. = The futon flew.

Sore wa sarada no sarada. = This is the salad plate.

Sake ga sakenda. = The salmon was shouting.

Hokkaidô wa dekkaidô. = Hokkaido is big.

Taiyô ni sawaritaiyô! = I want to touch the sun.

Share wa yamena share. = Stop telling jokes!
(This is often chanted menacingly when an adult pleads with the children to stop, just stop.)

Naiyô wa naiyô! = There is no meaning.
(This is the desperate cry of a weeping adult before the onslaught of children's jokes.)