The story begins with the three children (two females, one male), about ten years old, carrying the corpses of their parents to the back yard. Later you discover that they have exterminated all of humanity and that they have the power to kill any living being just by mentally desiring it. That is not their only power: they are also pyrokinetics and telepaths.
They have sex with each other and one of the sisters gets pregnant. They discover that soon the fetus is conscious and has mental powers superior to those of the parents. The story ends with one of the children expressing his fear of their descendant.
I'm not sure of the date of the tale, but I suppose it's about 1965-1970.