Instructions
Give your kids a piece of paper each, and write a sentence on a piece of paper or chalkboard that they can all see. For example, you could write "The black cat tripped while it jumped over the mat."
Have your kids write a new sentence that is exactly the same as the one you wrote, but with a different first noun.
Have your kids write a new sentence that is the same as the one they wrote above, but with a different first verb.
Repeat the above step, but have them change the adjective.
Repeat this for the second noun, second verb and second adjective.
Have your children change the first preposition.
Have your children read their sentences out loud, which should be very different from your sentence and hopefully funny in their mixed meanings. You could do this at any stage you want (hopefully, before they've lost interest).
Repeat the game, but let one of the children call out which word is going to be changed (first noun, second noun, etc).