Debt Stick

"The Chamberlain family died in a terrible car accident. No one survived. So thought the police…
Thirty years later, their relative Suzanne receives news: the remains of one of the children were found in the forest, who, according to the examination, lived for another four years after the accident. A mysterious stick with notches was found with him.
How could the child have survived away from civilization, among thickets and rocks? Did anyone else manage to escape? And why did he need the debt stick?"