Parents will be most resourceful here, perhaps. Or think back to when you were a child. Tell jokes that you once ( or current children ) thought/think funny.
They are often funny in a recursive sort of way by being ....not funny. Something like "Plan 9 From Outer Space" etc. but different. Sometimes they show originality ( "out of the mouths of babes " ) and sometimes the fact that the child thought that it was funny illustrates a phase they are going through, a new level of maturity.
A child was asked to complete this bromide. "Two heads are better than______" ??????
The child's answer - "None."
Remember long ago, if it was yourself, or, when kids try to tell dirty jokes for the first time ?
What about corney old jokes like "What's the biggest pencil in the world ?"
"Pennsylvania" Kids go through a stage where they really like stuff like this, and then they discard it. What is the next stage of humor for them ? How do they go about deciding to transition to the next level ?
Is it related to them being able to enjoy a game even more if they know there is a chance of losing it ? When they are especially young, sometimes they can't stand to lose, and so parents "playing" a game with them are challenged to make it more than just going through the motions, but yet still having to subvert it somehow.
Old people can be like this, too. I remember when I bartended at this one place, an old codger would often show up and ask me a riddle. I don't know if he just liked playing with me, or if he was truly that forgetful.
It was always the same riddle.
He'd ask - "Who's the king of corn?"
I'd say "You got me. Who is it, John ?"
"POP Corn !!" he'd say, and then laugh for a few minutes like it was hilarious.
He tipped well, and so I always went along with it to some degree. Sometimes someone else would "do the honors". It was like hearing a familiar pinball machine noise or something. Just part of the night noises.
An old street bum's joke to me one night - "Candy tastes good when you aint got no teef ! Ha Ha Ha "