Now, after an internet campaign to convince Rowling to personally meet Miss Cassidy, the author has written a personal note of comfort in the style of Dumbledore.
She also posted a wand, an acceptance letter to Harry Potter's school Hogwarts, a list of school supplies and a signed book.
A post on a Facebook page, trying to rally support for a meeting between the two, stated: "This afternoon, I talked to a friend of Cassidy's who confirmed that JK Rowling did, in fact, write her a personalized letter from "Dumbledore" (hand-written with purple ink).
"She was also sent a wand, an acceptance letter to Hogwarts with a school supply list, along with the 3rd book with JK's autograph.
"I'm so excited and ecstatic that we were all able to make a difference! How wonderful."
Rowling is said to have been so moved by seeing Miss Stay on global news sites that she tracked down her contact details and got in touch.
The Stay family, with the exception of Cassidy, was killed on July 9th. Father Stephen, 39, and mother Katie, 34, from Spring, near Houston, were gunned down along with Bryan, 14, Emily, nine, Rebecca, seven and four-year-old Zach.
Ronald Haskell, 33, has been charged with capital murder.
Cassidy survived because the bullet meant for her hit her finger and then grazed her skull, allowing her to play dead before she alerted police that the gunman was en route to kill her grandparents too.
Only three days after the massacre, Cassidy spoke to a memorial gathering at a local school, an event which touched the nation.
She told the crowd: "I'm feeling a lot better and I'm on a very straightforward path to a full recovery."
Wiping away tears, she added: "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'.
"I know that my mom, 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."
News of Rowling's gift was welcomed on the Facebook page, which has nearly 5,000 supporters.
One fan, Brindi Rawlings, wrote: "For such a busy person, I think it's awesome she took the time to write a personal note. Way to go JK."
Another, Kate Fitzgerald, added: " I think what Ms Rowling did was just right, warm and generous while respecting Cassidy's grief.
"This poor child needs some time to grieve away from the spotlight, which I'm sure JK understands better than most."
A spokesman for the 49-year-old author said: "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 is on the cards, she added: "We wouldn't comment on that but [Cassidy's] is a remarkable story."
The famous line quoted by the youngster is spoken by the boy wizard'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.
Appearing in the film based on the novel, the Prisoner of Azkaban, it is credited to writer Steve Kloves who transformed all seven books into screenplays.