In Mary Shelley's classic novel, Frankenstein or the Modern Prometheus, we do not get the details of William's deadly encounter with the creature until the monster shares the details with Victor.
When the monster travels to Geneva to find the creator who abandoned him, he finds himself on the grounds of the Frankenstein estate.
In Chapter XVI, the creature notes the boy (William) passing by. He reasons that he might be able to make the boy his companion because the child might be...
...unprejudiced, and lived too short a time to have imbibed a horror of deformity.
So the creature grabs William, however the boy is terrified:
'Let me go,' he cried; 'monster! ugly wretch! you wish to eat me and tear me to pieces—You are an ogre—Let me go or I will tell my papa...hideous monster! let me go...My papa is...M. Frankenstein—he will punish you. You dare not keep me.'
...The child still struggled, and loaded me with epithets which carried despair to my heart...
It is only through the creature's words that we learn of the last moments of young William's life.
Source: Shelley, Mary. Frankenstein or the Modern Prometheus. New York: Dover Publications, Inc., 1994. (p. 102)