Monsters are not real, but being afraid of monsters is very real. Because monsters are not real, it is important that adults don’t talk about them or do things that say they are real (slay them, cast spells on them, chase them away, etc.). See Should We Slay the Monsters?
What children are feeling about monsters (fear, anxiety, anger, etc.) is very real. Your children need to know you believe they are afraid. It’s OK to comfort your children and help them find ways to comfort themselves, while at the same time you are telling them monsters are not real.