"When do all the other kids come home?"
Her dad, Erik Rees, explained: "These children have different diagnoses than you do. Some stay days, some stay weeks, some stay years."
Then Jessie asked, "How can we help them?"
Back home in Rancho Santa Margarita, Calif. later that day in March 2011, Erik caught his daughter answering her own question by decorating brown paper lunch bags and stuffing them with Beanie Babies, activities, books and other gifts.
"I really want to help these children," Jessie said.
To keep the gifts sanitary for the hospitals, the family (including Jessie's brother and sister) switched over to 64-ounce jars that they named JoyJars, for Jessie's middle name
Around the same time, Jessie started posting videos on Facebook, encouraging other cancer kids to "never, ever give up."
This statement, NEGU for short, became the mantra of the Jessie Rees Foundation, which has distributed more than 80,000 JoyJars across the globe to 280 children's hospitals in 18 countries.
The project "started on our kitchen island and it quickly grew to our garage," says Erik, 44.
Courtesy of The Jessie Rees Foundation
"I have something for you, Cade," Jessie said as she handed him a jar stuff with Matchbox cars, Superman and Avengers action figures, crayons and a miniature football (each JoyJar is tailored to the recipient's interests).
The gesture left Cade "elated and excited" and also left his mom "so humbled and just blown away that this little girl had such a heart for others, including our son," says Cade's mother, Erin Spinello, 39, of Ladera Ranch, Calif.
Their family participates in Joy Drives and charity walks, and stuffs gift packages at the JoyJar factory in Rancho Santa Margarita.
Cade, who wears a NEGU wristband, says Jessie inspired him with her gift and her message.
"When things get hard, I just think about NEGU," he says.
Courtesy of The Jessie Rees Foundation
"She had an incredible foresight and vision to reach other kids at an emotional level," says Andrews, 49, of Coto de Caza, Calif.
Jessie, who swam with the Mission Viejo Nadadores Swim Club private swim team, also inspired Olympic gold medalist Kaitlin Sandeno, who had herself photographed with "NEGU" painted across her body and became a national spokeswoman for the cause.
"I go to hospitals and pass these out. And the nurses start crying and saying, "Nothing like this exists, providing this kind of care for these children,' " says Sandeno, 30, of Newport Beach, a fellow swimmer.
"And the most amazing thing about this movement was that it was started by an 11-year-old girl," she says.
In Jessie's MemoryBefore she lost her cancer battle on Jan. 5, 2012, at age 12, Jessie personally stuffed roughly 3,000 JoyJars, and included a personal note in each jar.
"She had three rules," her dad recalls. "No air – the jars had to be absolutely packed. No cheesy toys. And get them out in a timely manner."
In her final weeks, Jessie was also finding cancer-stricken children through Facebook who didn't have a lot of friends, and encouraging her own followers to pay attention to those children.
Jessie called this the "Joy Mob."
"This wasn't anything that my wife and I asked her to do," Erik recalls. "We just wanted her to enjoy life and she chose to enjoy life by helping others."
The mission could have slowed after Jessie's death, but her supporters, buoyed by her wish that they never, ever give up, have helped make the cause bigger than ever.
"To do something for others when you're feeling great is one thing," said Erin Spinello in explaining the charity's appeal. "But when people reach out to others in the middle of their pain, it is a privilege to be around that and I think it’s contagious."
Jessie's Facebook journal also lives on, with entries from her father, who on Dec. 12 posted a photo from two years ago – their last Christmas together.
"Last Friday, I was told an inspiring story by a Facebook fan from Mozambique, which is in Africa," he wrote. "She was working with a refugee there that was have a hard day. She shared your story with him and wrote in the dirt N-E-G-U... it’s that amazing, Jess."
Know a hero? Send suggestions to firstname.lastname@example.org. For more inspiring stories, read the latest issue of PEOPLE magazine