This is a short I heard about from a friend. Although it might have just been one of his fantasies.
The story involves a brother and sister about 8-9yo engaging in incest with each other. One scene has the boy and girl at the beach swimming, they come in to dry off then the boy starts touching his sister sexually.
The title he described to me is something like. "Terrible Children" or "les enfants terrable", forgive my poor french.
As I said this could just have been one of my mates fevered dreams, but it would be nice if he was telling the truth.

I looked up Les Enfants Terribles and came up with a 1950 movie that is not what you are looking for. Sorry.

Joey