When it comes down to it, the ending of Childhood's End is all about the unknown. Humanity has joined an unknown species named the Overmind to venture into the unknown reaches of space for an unknown purpose.
All that is familiar is destroyed: The Earth explodes and becomes food for the new species; civilization is kaput; and people as we know them—five fingers per hand, two eyes, the works—become extinct.
We don't want to oversimplify things, but the word unknown really does do an excellent job of summarizing this dense final chapter.
How you'll interpret the ending—happy or sad, triumphant or tragic, an act of deliverance or damnation—will depend largely on how you read the novel up until the end, and the answers you come up with for the many unanswered questions scattered throughout the text. But that's what the unknown is ultimately all about: exploring things you might not have the answers to and figuring it out for yourself.
The novel forces you to become a Jan Rodricks or Jeffery Greggson. Just like those characters, you too must venture into the unknown hanging out at the end of this book and see what you find there.
Clarke the Revelator
The novel is playing off a John of Patmos vibe with this ending, but like most of the Christian imagery in the novel, there's a twist to the proceedings.
Many have interpreted the Book of Revelations in the Bible as a prophecy detailing the end of the world. While a true study of Revelations could easily take up an entire college seminar, there are two important aspects we need to draw out of the biblical book for our little discussion.
The first is called the rapture, an event that is supposed to occur around the Great Tribulation when the believers of Jesus Christ will rise to heaven.
The second is the end of the world, sometimes called Armageddon. We should note that while the word Armageddon has come to generally mean the end of all things, it is traditionally thought of as the specific place where the final battle between good and evil will take place during the end times.
Anyway, back to the novel. The scene where humanity transcends to the Overmind has several imagery and thematic links to the Bible and the Book of Revelation specifically. Consider:
"Yes—I might have guessed. There's a great burning column, like a tree of fire, reaching above the western horizon. It's a long way off, right round the world. I know where it springs from: they're on their way at last, to become part of the Overmind. Their probation is ended: they're leaving the last remnants of matter behind." (24.36)
The destruction of the world harkens to Armageddon and the transcendence to the Overmind reads an awful lot like the rapture—just not a religious one.
Further biblical imagery can be found in the pillar of fire, which is similar to the image of Elijah, a prophet of the Old Testament, ascending to heaven in a whirlwind of fire. The fact that it looks like a tree also suggest the religious imagery of the Tree of Life mentioned in Genesis and Revelations both.
Even the phrase "leaving the last remnants of matter behind," echoes Ecclesiastes, while the destruction of the Earth is witnessed by Jan as a light "growing brighter, brighter, blinding"—and Jesus and God are both often depicted as forms of light throughout the Bible.
Yet the novel is playing off all these images ironically. The human race isn't being raptured to a heavenly afterlife but joining an alien race. And the Earth isn't being destroyed in a battle against the forces of evil to usher in an eternity of peace—it's being consumed by the newly formed Overmind to sustain itself the same way mother's milk keeps a baby healthy.
It's as though the novel is saying the religions of the world are right in saying the world will end and in the general imagery they employ to depict that event; however, the end result will be one of science and natural forces, not supernatural ones. The religious answers are correct save for the fact that they are answering the wrong questions.
Karellen: Overlord No More
But what about Karellen and the Overlords? Although we didn't get to know them too well, they still deserve an ending.
Like humanity, the Overlords also leave for destinations unknown. They know they'll have to serve the Overmind again, but they don't know where, for what purpose, or if they will succeed.
The alien species also hopes to one day overcome the dead-end that is their evolutionary path, though they don't know if they will succeed.
The one thing we can say for certain is that Karellen, one-time ruler of Earth, is no longer lord of mankind in its new form:
For a long time Karellen stared back across that swiftly widening gulf, while many memories raced through his vast and labyrinthine mind. In silent farewell, he saluted the men he had known, whether they had hindered or helped him in his purpose. (24.60)
It's no coincidence that the widening gulf is composed of outer space—what separated Karellen from mankind was the Overlords' mastery of space. Now, no longer the galactic children they once were, mankind has taken to the stars that Karellen once forbade them to enter. And arguably for the first time in the novel, Karellen respects man for its accomplishments.