The sound of unseen children

I headed along to the post office near my flat this afternoon to pick up a much delayed parcel (it arrived at last!). Along the way I passed by a school, or what I assumed was a school. The building itself was invisible behind several rows of thick hedges and a variety of other shrubbery. The only clue as to it's presence was the disembodied voice of children floating through the air. It's a generally happy sound, the noise children make whilst playing, but without the physical presence of a child to connect it with, it takes on an almost sinister quality, enhanced in this case by the absence of other pedestrians and a lack of traffic on the normally busy road I followed. What is it about kids that makes their rhymes, games and accouterments seem so eerie when taken out of their natural context? Is it just a result of some form of Pavlovian conditioning from watching horror films over the years, or is there really something about children that's inherently scary?