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.
When you buy a house, closets are just there. You could take the doors off, I suppose, but closets are just a thing. As for the void under the bed, that is a feature and not a bug. Yes, it may allow for a fear of what’s under the bed. It’s also an inherent defense against the actual bugs on the floor that would otherwise crawl up the child’s bed.
Basically, the answer isn’t to change the standard child’s bedroom but to instead work around it. Take the doors off the closet if it scares them. Check under the bed every night and maybe put their “bravest” stuffed animals under there for protection. But rooms are rooms. Blocking off a closet and putting the kid’s mattress on the floor is not the answer lol.