Lionel Messi is Argentine but holds a Spanish passport and gave birth to all his sons in Barcelona Spain, hence they are Spaniards by birth and holds both Spanish and Argentine pass, and could represent both countries internationally if they play football just like their father.
Cristiano Ronaldo Jr was born in the United States but lived his early years in Madrid, Spain while his father was playing for Real Madrid. He also played in the Real Madrid academy until Ronaldo left the Spanish capital for Juventus. As such he is eligible to play for Portugal, Spain and the United States.
Both sons of the world's two greatest footballers are progressing through the youth teams of the club's their fathers have played for. Cristiano Jr is currently in the Manchester United academy after his father moved while Messi's sons are in the Paris Saint-Germain academy after his shock move.
The two young players could be united as teammates at one point or the other in their careers if they both choose to represent Spain internationally, something that never happened to their fathers. Though the scenario looks very unlikely as Messi cherishes Argentina so much that he turned down the offer to play for Spain.