It is perfectly logical for a child to believe in monsters.
Most things she experiences are the first time she’s experiencing them. That something hasn’t happened to her yet is meaningless; most things haven’t happened to her yet.
She has *no way* of recognizing what is probable. *Everything* is probable. New, unimagined things happen every single day. The fact that something hasn’t yet happened to her has zero bearing on whether or not that thing normally happens.