By flashing forward to show what happens in the future.
Explanation:
The author creates the tension by quickly showing what will happen to the family that Scrooge and the ghost are observing. That family will lose the child, because the ghost tells Scrooge that Tiny Tin will die in the future and that next Christmas that child will not be alive and partying with his family. This death creates tension not only in Scrooge but also in the reader who wants the future to be modified.
