Bethany Fenton, who lost her battle with a brain tumor on Tuesday, was granted her wish to visit the Britain's Got Talent studios in North London on Saturday, where she sang to Cowell and met the finalists in competition.
"Simon let me sing 'Twinkle Twinkle Little Star' to him. He's so nice and I love him," Fenton told the Daily Mail after meeting Cowell.
Cowell's spokesman, Max Clifford, said the former Idol judge was "so sad" upon hearing the news of Fenton's death, and that he was extremely "touched when she sang for him."
"He is really upset; he said what a beautiful little girl she was," Clifford said. "He was happy that he was able to do a special something on Saturday night to give her a few precious moments, but it is so sad and his heart goes out to Bethany's mum and dad and he is thinking of them."
Robin Thompson, a family friend, added that Bethany had finally done what she always wanted to do. "She had achieved her big ambition and was now ready to depart this earth," Thompson said.
Calls to Britain's Got Talent's studio and Cowell's rep were not immediately returned.