The Purple Heart is a military medal traditionally given to soldiers who are wounded or killed in service. According to victim's family, the medal was sent in the post with a note that read: "The only heart I could find – be strong!"
"The family has been so touched and moved by this act of kindness that they would like to personally thank the individual, publicly or privately, for the gift and for their sacrifice for our country," the family said in a statement.
People have donated more than $50,000 to cover the medical expenses of the young victim, with thousands of well-wishers sending purple heart-shaped cards to her home in Waukesha, Wisconsin.
The girl's alleged attackers now face attempted murder charges. The pair of 12-year-olds lured the victim to the park after reading about Slender Man online, believing that her death would act as a sacrifice to the internet-born child stealer.
Slender Man first hit the internet on the Something Awful forums, after being created by the user Eric Knudsen. In a podcast earlier this year, Knudsen said that he "liked the idea of a creature that caused general terror and unease", but spoke out after the stabbing. "I am deeply saddened by the tragedy in Wisconsin," said Knudsen. "My heart goes out to the families of those affected by this terrible act."
If convicted, the two alleged attackers will face up to 60 years behind bars if tried as adults. Earlier this month, two doctors declared one of the girls unfit for trial and another competency hearing has been arranged for 1st August.
Follow Thomas Gorton on Twitter here @angstromhoot