Hi, while reading your post, I remember my son when he was little. He was afraid of the dark room, he said there’s a monster. I explained that it’s only your imagination, but still he was scared, eventually, he believed me.
I found this good article that says “Childhood fears are normal. Having those fears is a good opportunity to help them practice facing their fears — and practice determining if those fears were based in reality or their imagination."
http://bit.ly/2lwG7zX
It helped me in dealing with my son’s fear, it might help you too.
Thank you for sharing.