This is the 418th most frequent Finnish word.
lapsen
"Lapsen" means "child's" or "of the child" in English.
In this context, 'lapsen' is in the genitive case, linked to 'elämästä' to indicate whose life the story pertains to.
Kuulin tarinan lapsen elämästä.
I heard a story about the child's life.
Indicates possession (child's toy).
Lapsen lelu on pöydällä.
The child's toy is on the table.
Indicates the object of action (the child is being seen).