"That was so amazing. It made my day." Clarkson said in a video sent to the kids on Wednesday. "It was so meaningful…Thank you so much, I just can't wait to meet you."
The "Stronger" video, which has had more than half a million hits on YouTube since it was posted on Sunday, shows the young patients, many of whom have lost their hair during cancer treatment, lip-syncing and dancing with nurses on the hematology-oncology floor to Clarkson's song.
"They are beyond excited to see Kelly's response," Louise Maxwell of the hospital's public affairs office told ABC News. "Their feet haven't touched the ground."
An emotional Clarkson holds back tears in her video. "I'm not going to cry because I have to go perform right now. It was just so beautiful."
A new video shows the young patients as they watch Clarkson's response. The kids seem floored, all yelling, "Thank you Kelly!"
One young girl adds, "You might have to have a concert."
The idea for the original "Stronger" video came from a 22-year-old cancer patient, Chris Rumble, who created the video as a response to a birthday message from his former hockey team.
"I wanted to make a video to send back to my team and I thought what better way to do it then with the kids on my floor," Rumble said to the Seattle Children's Hospital "On the Pulse" Blog.
Rumble, an aspiring filmmaker and hockey player, was diagnosed with leukemia in April after a doctor discovered he had swollen glands. Rumble was immediately taken to the children's hospital where he began an aggressive treatment plan.
"The diagnosis hit me like a brick wall," he said. "I didn't have time to be sad because everything just happened so fast."
The video was created for hospital's "Not Now" program, which aims to help adolescents and young adults cope with cancer by providing creative projects to connect with their peers, according to Maxwell.
As a fan of Clarkson's, Rumble said he thought the song was perfect for relating to those battling cancer.
"It was not only good to see the kids happy, but it was also great to see how their parents were so happy as they watched their kids just being kids - dancing, singing and having fun," Rumble said in the blog.
Maxwell said the making of video was an uplifting event for the kids and nurses on the floor: "It was a fun, happy thing to do; a great distraction to the kid's treatment."
But no one expected the outpouring of support from the public and Clarkson herself.
"They never expected this," Maxwell said, "The video from Kelly, emails, and comments on YouTube are wonderful expressions of support for the kids to hang in there and stay strong."
The clinical director of Seattle Children's Cancer and Blood Disorders Center, Dr. Douglas Hawkins, added, "It has been this incredible morale-booster to have this video get such widespread attention across the country and across the world."
"It's so reaffirming for everything we've been going through," one parent said.
A young patient added, "The support that people are giving and all the nice things they are saying, it just really helps. It's very nice to hear."