Childhood's End originally began as a short story called "Guardian Angel." The story basically consisted of the first half of the novel, until the point at which the Overlords reveal themselves. The original concept behind the Overlords, in the short story, was not that they were shepherds for the human race. The point of "Guardian Angel" was simply the irony of having these beneficial aliens turn out to look exactly like our conception of the devil. Only later, when Arthur C. Clarke was pushed by his publisher to expand "Guardian Angel" into the full novel that would become Childhood's End, did Clarke introduce the idea of the Overlords as shepherds watching over the next step in human evolution.
There are two ironic conceits in "Guardian Angel." The first is that Clarke takes the concept of an alien invasion, already a cliché in 1953, and turns it on its head: the Overlords come down in their ships over all the major cities and, instead of blowing them up, they turn Earth into a utopia. The second conceit is that these friendly aliens look exactly like a medieval conception of Satan. "Guardian Angel" was about challenging preconceived notions and the nature of prejudice, and the Overlords are the main symbol of that theme.
In Childhood's End, the Overlords become much more than ironic symbols. They become tragic figures, forever stagnant, helplessly subservient to a transcendental force much more powerful than themselves. They must watch time and again while other races achieve the next step, crossing the galaxy and even the universe with near-omnipotent powers, while the Overlords continue to zip around in their spaceships. Childhood's End is perhaps one of the most ironic science fiction novels ever written, and there is no better symbol of that irony than the Overlords, who at first seem so radically superior to humanity, but truthfully envy it.