Carrie Underwood's five-year-old son is "so proud" he's part of her first ever Christmas album, he's playing the sweet song to all his school pals.

Isaiah, who the singer shares with husband Mike Fisher, contributes guest vocals on Little Drummer Boy, and he couldn't be more excited to be part of mum's festive collection, titled The Gift.

"I had no idea what was going to happen (in the studio)," the American Idol winner told Kelly Clarkson on her daytime talk show of her kid's big performance.

"He's never been in that kind of situation before. I don't think he's even been like, in the studio with me ever before."

In fact, the Love Wins hitmaker isn't even sure Isaiah knows what his mother does for a living.

"I don't know if he knows what I do at all," she laughed, "but I could hear his voice on it (track) in my head when I was singing it in the studio and I just thought, 'Ask him and see if he might want to (sing on it)'."

And she needn't have worried if Isaiah would rise to the occasion because the star rated her child's performance as "great".

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