I know it was Joe Jr who was groomed, and then JFK took his place after Jr's death.
"From a very young age, Kennedy was groomed by his father and predicted to be the first Roman Catholic US president. When he was born, his grandfather John F. Fitzgerald, the mayor of Boston, told the news: "This child is the future president of the nation." He often boasted that he would be president even without help from his father." Wikipedia:Joseph_P._Kennedy_Jr.
I was thinking maybe John Quincy Adams but I don't know much about him.
Also, maybe Bush 41, 43 and Jeb Bush?
I'm also interested as an aside if there were any other near-presidents, esp VPs or governors like Jeb who were groomed from birth or childhood to be president.