Robert and Bindi Irwin said they would remember The Crocodile Hunter in their own personal ways to mark the occasion on Tuesday, sharing old photos online.
Both have followed in their father's footsteps by becoming environmentalists and television personalities.
Irwin, known for his fearless confrontation of dangerous beasts around the world, died in September 2006 after being pierced in the chest by a stingray barb.
Robert said that every minute spent with his father had been "a blessing".
"Happy birthday Dad. Love & miss you every day," he wrote.
"My earliest memories are my fondest thanks to you. Hanging on to the handlebars of the motorbike, on your morning lap of the zoo.
"Every minute with you was a blessing. I'm gonna ride that lap on your old motorbike this afternoon."
His older sister, who was eight years old when their father died, wrote: "Happy Birthday to the greatest dad and Wildlife Warrior.
"Today I'll watch The Crocodile Hunter with Grace Warrior and share stories about her incredible Grandpa Crocodile.
"I love you with all my heart. Your legacy lives on."
Ms Irwin, is married to American former professional wakeboarder Chandler Powell, with whom she shares a daughter, Grace.
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