Why do people find singing children creepy?

I know what you mean. For example, a scene filmed in black & white, where there are a group of children singing 'ring a ring o' roses' and dancing around in slow motion.

Now, in that particular example, the song that goes "Ring a Ring o' roses, a pocket full of posies..."(which is often used in those creepy scenes) was once thought to be about a deadly disease (the plague) which is surprising given it is a children's nursery rhyme. The words in that song, were originally thought to have been written to describe the stages of disease that a plague victim would endure (a circular ring-like rash, sneezing and breathing difficulties, and then death.) A lot of people now say the nursery rhyme has nothing to do with the plague (owing to inaccuracies in when the song was first heard/created), but for a long time, film makers would use the song due to it's alleged links to disease and death.

Apart from this rhyme however, there is still the question as to why a group of children singing is found to be creepy (regardless of the song they are singing.) I think that is a fairly modern phenomenon (much like how clowns have only recently become used as horror-figures.)

My explanation would be that one of the main methods that is used to make horror films scary, is dramatic tension. While someone 'jumping out' and scaring someone, will provide a short-fright, it is easier for film makers to frighten their audience, by extending and drawing out a scene where the viewer 'knows something bad is about to happen' but they just don't know when.

The classic 'creepy singing children' serves the purpose of providing a 'gauge' against which the films frightening/gory scenes, can be measured. Metaphorically, children represent innocence and playfulness. They provide very little threat. They are the opposite of evil, even up to the point that a common phrase used to describe a child is 'a little angel.' The audience knows that something unpleasant is going to happen in the horror film, and the 'singing children' offers up ideas of safety and happiness, that will make the films scary scenes, seem even worse in comparison.

An additional explanation is many horror story plots revolve around the concept of an evil witch or demon capturing children (or an evil force that targets children.) Humans are naturally protective of children, and when we see children dancing around in horror films, it is done to trigger our natural sense of wanting to protect those who are weaker than us. We see the children dancing (and perhaps some creepy music playing) and we think to ourselves "Oh no! Surely the demon/witch/evil-spirit/ghost isn't so nasty that he/she will be cruel to the children?"