The Post quotes her mother, Sage, as saying that Sadie, now in fifth grade, wondered why the transgender community wasn't included.
"The world would be a better place if everyone had the right to be themselves, including people who have a creative gender identity and expression," Sadie writes in her letter, entitled "Sadie's Dream for the World."
"Transgender kids like me are not allowed to go to most schools because the teachers think we are different from everyone else," she continues. "The schools get afraid of how they will talk with the other kids' parents, and transgender kids are kept secret or told not to come there anymore. Kids are told not to be friends with transgender kids, which makes us very lonely and sad."
Sadie says it would be "a better world if everyone knew that transgender people have the same hopes and dreams as everyone else."
The full text of Sadie's letter is published on TransGriot.
The Post notes that Sadie, who "socially transitioned from male to female in kindergarten," listens to Lady Gaga, Pink and Justin Bieber and wants to work for Greenpeace.
Sadie's mother told The Huffington Post that while her daughter has faced discrimination, "she isn't shy or ashamed of who she is."
"When she chats with people, she introduces herself as, 'Hi, I'm Sadie, my favorite color is pink, I'm vegan, and I'm transgender. Who are you?'" her mother said in the report.
Landing image: Huffington Post (photo from Sadie's family)