The handwritten note of comfort, written in purple ink, was accompanied by a wand, an acceptance letter to Harry Potter's school Hogwarts, a list of school supplies and a signed book.
A spokesman for the 49-year-old author told the newspaper: "We can confirm that JK Rowling was in touch with Cassidy Stay, however, the contents of the letter remain private."
Asked whether a meeting between the two may happen, she added: "We wouldn't comment on that but [Cassidy's] is a remarkable story."
At the memorial Cassidy said: "In The Prisoner Of Azkaban, Dumbledore says, 'Happiness can be found even in the darkest times if one only remembers to turn on the light'.
Cassidy Stay speaks during a memorial service for the Stay family at Lemm Elementary School. Photo / AP
"I know that my mum, dad, Bryan, Emily, Becca, and Zach are in a much better place and that I'll be able to see them again one day. Stay strong."
The line quoted by Cassidy is spoken by Harry Potter's mentor Dumbledore - played by Sir Michael Gambon - as he addresses the pupils of Hogwarts in the Great Hall at the start of the film The Prisoner Of Azkaban.
The quote is credited to writer Steve Kloves, who transformed all seven books into screenplays.