Carrie Underwood Says Son, 5½, Told Everyone at School About Their Duet: 'So Proud of Himself'

Carrie Underwood's 5½-year-old was beyond proud of his mommy-son duet — and he let his friends know it!

For her new Christmas album My Gift, the country star, 37, enlists her son Isaiah to help her sing "Little Drummer Boy." During an appearance on The Kelly Clarkson Show on Friday, Underwood says her older child (she also shares son Jacob, 2 next month, with husband Mike Fisher) was telling all his classmates about his song debut.

"I had no idea what was going to happen," Underwood says of asking her child to sing with her. "He's never been in that kind of situation before — I don't think he's even been in the studio with me ever before. I don't know if he knows what I do at all. I just thought I could hear his voice on it in my head when I was singing it, and I thought I'd just ask him and see if he might want to."

"He did such a great job, and listening back, I think he's just so proud of himself," she continues. "He went to school and he's like, 'Hey, my song's out! Wanna hear my song?' And they were listening to it in his class."

"I think it's adorable," says Clarkson. "And it's cool that I think, when we're singers, our children take some kind of interest in what we do. That's kinda cool, 'cause we work a lot."

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Underwood recently told PEOPLE that it was "so great" to perform with her son.

"When we recorded, I was able to be in the booth with him and encourage him and try to remind him what words were coming up next, things like that," she said, adding, "I got to listen to his vocals [get] put together and then listening to myself with him. I was laughing, and I was crying, and it was just very overwhelming in a good way. It's just very emotional."

During an appearance on PEOPLE (the TV show!) in October, the American Idol alum said that when it comes to her son realizing that his mother is a famous artist, she can "see little lightbulbs coming on" in him.

"It's mainly when other people talk about it to him," Underwood explained. "If we'll be there with a little friend and the friend's mom is like, 'Oooh now this is Carrie Underwood.' "