The tone of Childhood's End sounds like a scientist discussing an experiment he performed to a seminar of his co-workers. Here's a sampling of what we're talking about:
In many ways, the appearances of the Overlords had raised more problems than it had solved. Their origin was still unknown, their biology a source of endless speculation. On many matters they would give information freely, but on others their behavior could only be described as secretive. (6.16)
Now remember, the entire first part of the book builds up around the fact that the Overlords will not show themselves to mankind. Stormgren and Wainwright argue over this fact, and Stormgren does everything he can to solve the mystery.
So you'd think that when the moment finally arrives we'd see how this grand unveiling affects people on a personal level. But do we? Nope.
The essential human aspects of the demonic appearance are absent save for a brief mention of a fainting woman. The narrator's tone hardly dives into the perspective of individual people. Were they scared, overjoyed, confused, enraptured or all of the above? We can't say.
Since the novel is focused on the big picture, the human element is kind of left behind. Instead, the narrator's tone treats the situation like an anthropologist looking back on an event. The tone is dispassionate, calculating, and focused on analyzing the facts rather than diving into the heart of the matter. For us, the phrase "biology a source of endless speculation" from the quote above really points to this quality.
Of course, this is a science fiction novel, so having a narrator that sounds like a scientist makes sense, doesn't it?