I read about this idea in a magazine 5 years ago while in my OB/GYN's office.  Unfortunately I cannot tell you the name of the magazine to give credit where credit is due. 

The way to monster proof the room is to get an empty spray bottle and fill it with water.  You can decorate the bottle, call the fluid inside whatever you want, but basically the "story" is that monsters won't cross over the water.   So when you are ready to tuck your child into bed, you spray by the window sill, around the closet, the door to their room, etc.  Wherever they feel a monster is going to come from, that is where you mist thus enabling the monster repellent to take effect.  If your child wakes up in the night concerned, you can have them spray the monster repellent again or you can do it for them.  I only had to monster proof her bedroom for a month or so, then that phase was over.  I was glad to have found this tidbit since it wasn only a few weeks after that my daughter started being afaid monsters.  Hopefully this can alleviate some stress for yourself and for your child.