I took my weekly trip to Michaels and found some cute monster stickers. I took an old Glade spray bottle, peeled of the labels, and decorated the bottle with the monster stickers and some letters I already had. Then I just added some water to the spray bottle. You could also add some essential oil if you wanted to add a scent but I didn't.
It seems to be working. So if your child is complaining of being afraid of monsters, maybe you need some monster spray also:)