I’m not a parent, but going by pop culture, it seems like literally every child has the same fears.
In pre-modern times, I imagine that they’d be sleeping in the same room as the parents, but if modern notions of privacy don’t permit that, seems we could at least design an enclosed capsule or something.
Kids are afraid of being alone in the dark. The monster under the bed or in the closet is just how they communicate their fear.
I had a room similar to what you suggested. No big closets and an elevated bed. But I still got scared sometimes. And the only things that helped were being in a well lit room and or not being alone.
That makes sense.