I wouldnt introduce the monster spray or any other monster specific protection. To say they dont exist and then have monster spray is a bit contradictory and if they are already worried about the presence of monsters I would think its just going to confirm that they are real.

I would keep re affirming that monsters arent real. The do not exist and there is nothing to be afraid of.

The books and movies, I think, are ok. Many books and movies are about make believe things, the important thing is to know the difference. I would point out that cars dont have eyes or talk, animals do not not talk like us and monsters and fairies are pretend.

Lots of cuddles, reassurance and honesty.