When my children were small they would worry about monsters in their closet or under their bed. Nothing I seemed to say would calm their fears. Then one day I read about "Monster Repellent." The recipe for monster repellent goes as follows.
1 squirt bottle
water
a bottle of scent (the cheaper the better)
Fill the squirt bottle with water and pour in some of the scent. Then spray the monster repellent around the child's room. On the bed, under the bed, in the closet, or any other place the monster likes to hide. The child will smell the repellent and remember that it is working keeping them safe.
The other day I had a student come to school crying and frightened by some monster he had seen on tv. I didn't have any repellent at school so I pulled out some glow gel that I use when teaching the children about washing hands. Anyway I put some of this gel on his hand and then flashed a black light on it. I told him that the gel would kill any monsters that came within a mile of him. I haven't had a problem since with his irrational fears.