An idea I saw on pinterest...not sure if it works but might be worth a try?? (works as in the child believing it...not that it actually keeps monsters away :P LOL)

Make up a bottle of "monster spray"....(I've seen other cute names too but can't recall them right now!)....just in a spray bottle, mix water with a scent. Lavender would be good since it is a calming scent. Then you and your child spray the room to keep "monsters" away!! To make it more real, you can print off a label to make it look more official and store bought!!