Of the 11 songs on "My Gift," the "American Idol" winner, 37, has duets with John Legend on "Hallelujah" in addition to the one with her eldest son.
“[Isaiah] loves to sing. He loves music. He definitely has that passion and I asked him, ‘Would you ever want to go in and sing with Mommy?’ He said, ‘Yes,’” Underwood told the Associated Press.
The country superstar -- who is also mom to her 1-year-old son, Jacob -- said she wasn't sure it would work but, in the end, "he went in and he did such a great job."
"He was throwing his hands up in the air and was so excited and had all these cute 5-year-old emotions just pouring out of him," she gushed.
Underwood called the sweet collaboration "just such a proud moment for me as a mom."
The "Find Your Path" author also said husband Mike Fisher, 40, was "just so excited" to hear his wife and son's version of "Little Drummer Boy" for the first time in the car.
"My Gift" also features two songs she helped write, "Let There Be Peace" and "Sweet Baby Jesus."
The seven-time Grammy winner is set to perform songs off her newly released Christmas album for an upcoming HBO Max holiday special.
Listen to Underwood's duet of "Little Drummer Boy" with son Isaiah here: