In honor of that special occasion, the country-music star performed a one-of-kind rendition of "Happy Birthday" for the boy, and thanks to Isaiah's dad, Mike Fisher, all of her fans can enjoy can enjoy it for themselves.
In a clip the retired NHL player shared on Instagram, it quickly becomes clear that Underwood wasn't about to belt out a classic take on the tune. The giveaway comes when she grabs a birthday balloon and inhales the helium right out of it.
That's when the high-pitched — yet somehow still perfectly pitched — verses start, and mom really delivers a melismatic performance for her little man.
Dad can be heard chuckling from behind the camera, but it's nothing compared to giggles coming from the birthday boy as he takes in the song without taking his eyes off mom.
If that version of "Happy Birthday" made the 35-year-old singer sound more like a Muppet than a country superstar, then it was fitting for the party's theme. In another post, she showed off Isaiah's Kermit-themed cake in honor of his favorite movie, "Muppets Most Wanted."
While Isaiah was the star of the party, he wasn't the only star in the clip besides his mother. Baby brother Jacob can be seen sleeping through all the fun while cradled in Underwood's arms.
It's a rare glimpse of the baby Underwood and Fisher welcomed to the world back in January — whom Fisher referred to as "a gift from our Lord" just after he was born.