Carrie Underwood‘s family celebrated a big day last Tuesday (Feb. 27): Her oldest son, Isaiah, turned nine years old.

To mark the occasion, the family of four planned a party centered around one of Isaiah’s favorite hobbies — taekwondo — and even commissioned a custom cake to match the theme. Underwood shared some snapshots of the finished product: A two-tiered cake decorated with colored belts, the ATA Martial Arts logo, Isaiah’s name and even a cake topper of a young boy, mid-high kick, wearing a taekwondo black belt.

“Today we got to celebrate our sweet Isaiah turning 9!!!!” The country singer wrote on Saturday (March 2.) “Oh, how time flies…”

Though she was feeling a little nostalgic, Underwood was mostly excited about the experience of watching Isaiah get to introduce some friends to one of his favorite activities. “So fun to see the kids learning some new things,” she continued. “And what an amazing cake from [Ivey Cake Store]!!! It was as yummy as it was cool!”

A love of physical activity seems to run in the family for Underwood’s and her two children. The singer’s younger son, Jacob, turned five years old in January, and he celebrated his birthday with a round of after-dark ice skating on the family frozen-over pond.

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Of course, the boys come honestly by their love of sports and physical fitness: Their dad is former NHL pro Mike Fisher, who played for the Ottawa Senators and, later, the Nashville Predators. It’s no secret that Underwood is passionate about physical fitness, either. She’s got her own line of workout clothing and her own fitness app.

She also continuously challenges herself with new physical stunts onstage. Most recently, the singer incorporated aerial stunt work into her show after being impressed by the aerialists booked to perform during her Las Vegas residency.