Exclusive: Inside Kate Middleton's Meeting With Cancer-Stricken Girl, 6

Duchess Kate's most unforgettable moment during the recently wrapped royal tour of North America may not have been her stunning red carpet debut in Hollywood or romantic asides with husband Prince William.

It was a much quieter, profoundly moving encounter following her July 7 arrival at the airport in Calgary, Alberta, when Kate, 29, was greeted by a six-year-old girl bearing a bouquet. In an unscripted moment, Diamond Ann Marshall, who suffers from stage IV undifferentiated sarcoma, broke with royal protocol and embraced Kate, who responded in kind.

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"It was a dream come true for her," Diamond's father, Lyall Marshall, tells Us Weekly. "She had always wanted to meet a real princess. They got along very well. At first Di ran over and hugged Kate's legs and then ran back to us after handing over the flowers, but then Kate kneeled down and made her feel welcome. They talked for a few minutes. [Kate] told us the flowers were beautiful. The Duchess was very kind to her, very sweet and genuine. "

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Diagnosed earlier this year, little Diamond watched William and Kate's royal wedding from her hospital bed at the Alberta Children's Hospital. Inspired, she hand-wrote a note to England's future Queen, which read:

"Dear Princess Kate,

My name is Diamond Ann. I am six. I was named after Princess Diana. My Mommy Memory is in heaven with her. I have cancer. I spend a lot of time in hospital. I watched you get married from my bed there. You looked pretty.

I like playing Princess dress up. My favourite princess is Aurora. Who is yours. I would really like to meet you. Do you wait [sic] to me too?

Lots of love,

Diamond xx"

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Thanks to the efforts of the Alberta Chapter of The Children's Wish Foundation of Canada, Diamond's letter made it into the hands of the royals. "We never in a million years thought the Duke and Duchess would hear our request," her father says.

After that once-in-a-lifetime meeting at the airport, Diamond arrived home in a jubilant mood. "She was walking around the house so happy, so excited. She just had her wand, smiling and laughing."

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Diamond (whose mother, Memory, died of cancer four years ago) is currently in and out of chemo sessions but "responding well to treatment and her tumor is shrinking," with surgery planned for mid-August, Marshall says. (Doctors first discovered tumors in her abdomen and lungs earlier this year.) And Duchess Kate has something more permanent than a bouquet to remember her meeting with Diamond. The little girl also gave Kate a heart-shaped box, with a friendship bracelet (made by Diamond) inside.

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"We were so happy she got to meet a princess," Marshall tells Us. "That was her dream and we are so thankful for the Children's Wish Foundation and the amazing work they do. It's been such a boost to her health and happiness."

Since 1984, the Children's WIsh Foundation of Canada has granted life-changing wishes -- granting nearly 3 per day -- to 17,000 kids aged 3-17 diagnosed with life-threatening illnesses.

Visit ChildrensWish.ca for more information.