Five-time Ball d'Or winner Ronaldo, 33, signed for Italian champions Juventus this summer from Real Madrid for 100 million euros.
His oldest son Cristiano Jr -- who was born in June 2010 -- was photographed training on Monday with the Juve Pulcini 2010 squad reserved for Under-9s at Juventus's Vinovo ground.
Ronaldo's partner Georgina Rodriguez was also present at the training ground.
Other senior Juventus players have children in the team's academy including Andrea Barzagli's son Mattia and fellow former Italian international Claudio Marchisio's son Davide.
Ronaldo revealed last week that Cristiano Jr -- the oldest of his four children -- was also a promising footballer.
"He's very competitive. He's like me as a child. He doesn't like losing," the striker said in an interview with streaming broadcaster DAZN.
"He'll become like me, I'm 100 percent sure of it. I'd like to teach him some things, but ultimately he will choose what to do and he'll always have my support.
"But of course, I'd like him to become a player because I think he has a passion for it too."