Mummy, do ghosts and monsters really exist?

I bought a story book for my kids called Mess Monsters. It tells the story about a little boy who always leaves his room untidy and never cleans up his mess. His mother has to keep telling him to clean up his stuff but he just leaves his books, toys, etc, all over the place. So one night, some monsters crawled out from under his bed and played with his stuff, throwing them around and breaking all his toys.
The monsters also kept disturbing the little boy. So the little boy pleaded with the monsters to leave him alone and not to destroy his things. He promised to clean up and will never leave his stuff around anymore. Upon hearing that, the Mess Monsters went back under the bed and never came out again.

After reading this story to my kids, they looked under the bed to check whether there are any monsters underneath. And when the left they toys on the floor, I reminded them that the Mess Monsters will come and destroy their toys. This scared them and they started cleaning up. So every time when they refuse to clean up, I will remind them of the Mess Monsters.

My daughter asked me whether there really are monsters and she said that she is afraid of the monsters.

What would you do? Tell her that monsters do not exist or continue with the Mess Monsters story so that they will clean up their mess?