Why do children have this primordial fear of clowns?
Thats a good theory now but Children have had a fear of Clowns before they were used in Movies and a lot of Kids have a fear of Clowns without having seen any movies featuring evil Clowns I did a quick bit of research into the history of Clowns and found the following from about.com which could explain things a little:
"Clowns date back to the jester or fool of ancient times. In those days, the clown was given permission, and even expected, to represent the deviant side of human nature, from openly defying the sexual norms of the day to mocking the gods. As time went on, the jester morphed into the trickster, a more sinister figure with intentions that were less than honorable.
The modern circus clown is an outgrowth of the tramp clowns of the Depression era. Tramp clowns were largely members of the "unsavory" underclass who entertained the more privileged with a caricaturized look at their daily existence. Although most tramp clowns were harmless, a seedy underbelly did exist among the clown circuit.
Weary Willie was the clown alter-ego of the legendary Emmett Kelly. While Kelly achieved stardom with his character, his personal life was a mess. His wife eventually filed for divorce, claiming that the character had taken over her husband's personality. Their son, Emmett Kelly, Jr., took over the role on his father's retirement. Kelly, Jr. took the character to even greater heights, but his wife, too, felt that Weary Willie eventually overtook Kelly, Jr.'s personality.
In the 1970s, two events occurred that may have fueled our collective fear of clowns. Paul Kelly, the son of Emmett Kelly, Jr., lost a leg in a train accident. Nevertheless, he eventually came to the conclusion that he needed to become the next incarnation of Willie. He began calling himself Emmett Kelly III and performing as Weary Willie. Simultaneously, he slid into a life of drugs and sexual freedom. In 1978, Kelly III was arrested for the murders of two of his homosexual partners. He admitted to the slayings, but listed "Willie" as an accomplice.
Eventually, Kelly III was diagnosed with multiple personality disorder. Willie had apparently taken over his personality as he had those of Kelly's father and grandfather. Although this sort of case appears to be isolated, and most clowns do not show deviant behavior, the case was reminiscent of the malevolent trickster archetype of earlier lore.
The highly publicized murders committed by John Wayne Gacy also took place in the 1970s. From 1975 to 1978, Gacy sodomized and killed at least 33 boys and young men in the Chicago area. Although Gacy never worked as a professional clown, he did perform on a volunteer basis. This connection has been played up over the years in movies and films about the crimes.
By the 1980s, clown phobia had reached a peak. Rumors of ritual abuse of children were rampant, and clowns figured heavily into many of the stories. Spontaneous reports of clown harassment began pouring in from children nationwide. Even urban legends began to focus on killer clowns lying in wait for hapless babysitters."