Fear Has Its Own Logic

A young child who fears the dark, monsters, or being left alone is not being silly. Between roughly ages two and seven, imagination grows much faster than the ability to test what is real. That gap is the whole story.

When you say "there is nothing to be scared of," the fear does not leave. It just stops being something the child says out loud. It goes underground and tends to resurface sideways: trouble falling asleep, sudden clinginess, a refusal to do things they used to enjoy. You have not removed the fear.