Before we discuss what's up with the title, we thought we'd share this famous quote, spoken by one Isaac Newton:
To myself I am only a child playing on the beach, while vast oceans of truth lie undiscovered before me.
Okay, we know what you're thinking: What does the father of calculus have to do with a science fiction novel about a passive-aggressive alien invasion? Well, Clarke kind of writes the exact same quote in Childhood's End. Check it out:
[…], yet sometimes it seemed to [George][ that men were like children amusing themselves in some secluded playground, protected from the fierce realities of the outer world. (16.7)
Sure, Clarke repurposed and repackaged the quote a tiny bit, but the essence of the two thoughts is the same, and both point toward what is up with the novel's title.
Childhood's End is all about, well, the end of childhood. Only it's not an individual person's childhood, but the metaphorical childhood of the human race. Earth can be seen as a type of nursery for mankind, and as the novel progresses, humanity begins to grow little by little until the nursery is too small for our species.
The Overlords come to confirm we aren't alone in the universe, Jan treks out into space to discover unknown worlds, and Jeff and Jennifer evolve (read: grow up) into something completely un-human. All of these occurrences speak to people discovering more and more about the universe around them and themselves. All of these events take humanity from its infant state and into those vast oceans of the unknown Newton talked about.
Also, take note that Jan begins his voyage into space from the ocean, and Jeff's transformation begins while he explores the ocean. Coincidence? We don't think so. Childhood's End is all about the end of childhood and the entrance into that vast unexplored ocean.