Originally begun in 1957 as an American picture to star Ronald Colman, MGM shelved the project, because it was deemed potentially inflammatory and controversial, specifically due to its sinister depiction of virgin birth. Colman had passed away by the time this film was slated for production in 1959; in an odd twist, his replacement was George Sanders, who had recently married Benita Hume, Colman's widow.
The novel this film was based on was called "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.
Except for one sequence, every time the glowing eyes are show, it's a still photograph. At one point, David--after the motorist almost hits one of them--is staring at the man, while a girl next to him is moving. There's a very visible jagged line separating the moving shot from the still frame.
The baby children were all played by Kim Clarke Champniss, who as an adult would become a very popular Canadian music journalist and television personality, especially familiar as one the original MuchMusic veejays.
When little David opens the Chinese box for the chocolate, his father allows a blonde girl to also open the box. In the meantime, her mother is standing off to the side with another blonde boy in her arms. It's obvious looking at all three children that they're played by the same actor. The little girl is wearing a very obvious wig, and their faces are identical.