The monkey in the bathroom.

Tell a child there is a monkey in the bathroom.

They will go look and call you a liar. Go with them in the bathroom and point at different places and claim you see it. They say whatever, usually loudly.

Then you say "Oh there it is!" ... and point to the mirror you have strategically placed them in front of. (sometimes it takes a chair, incorporate that in the joke as in "you need to be up higher".

Have fun :)