It took me a moment to really try to see what you were going for then i saw the bed post, actually almost got lost on my view. The monster is too flat to be a really frightening thing, if you had a doll or some other way to make just eyes appear from under the bed it would have conveyed a much larger sense of fear to me.
Aha! So THAT's what it looks like! This image has a nice, dark mood to it. The whimsy present in the obviously fake monster is an interesting choice. Perhaps putting this monster on a third, off center, would add even more drama and exaggeration.
The photo is too dark and a bit out of focus. Its also difficult to tell if there is even a bed there. Maybe someone lifting a cover to find the monster would help this scene.
The idea is good, a few technical improvements might have enhanced it more. For example it is way too grainy and I would have also gotten more of the bed into the shot for example.