I spent a productive hour with the third grader I'm working with at the school. World leaders will be relieved to know that we did not work on the Evil Albert Einstein story, so our villain did not get a chance to destroy the human race.
Instead, in an effort to understand what it feels like to actually finish something, we worked on two super-short stories. One was about a carnivorous computer attacking the house of a woman called Gate. She lures the attacker away from her house by sneaking out to the street and impersonating several Fat Old Ladies, the computer's preferred snack.
The second was about a man called Writing a Formal Piece (I think...that may not be quite right) (it's Formal for short, I do remember that) who discovers that his daughter, a cannibal alligator disguised as a girl, is about to kill his wife. He sees this as a problem because his wife had been planning to make him his favorite meal for dinner.
I don't think any of us should ever worry about whether our stories are too scary for children.