Based on the John Wyndham novel, "The Midwich Cuckoos". The title refers to the fact that when cuckoo birds lay eggs, they deposit them in the nests of other (unsuspecting) birds, who then raise the cuckoo chicks as their own. Compounding the insidious nature of this process, the cuckoo chicks often kill their nestmates in competition for food and parental attention.
The eerie effect of the children's glowing eyes was created by matting a negative (reversed) image of their eyes over the pupils when they used their powers. The British print of the film contained no optical effects, as the British Board of Film Classification considered them too frighting for an 'A' classification.
In the original novel, sixty alien children are born. The film whittled that number down to thirteen (including David).
When the military are testing Midwich for radiation after everyone wakes up, there is a sign in the background that says "Beware of Children." Another "Beware of Children" sign appears towards the start of the car crash scene.
The film rights were purchased even before John Wyndham's novel "The Midwich Cuckoos" was published.