Roger DIDN'T say "King of the pirates" here to Whitebeard and Oden. He attributes that title to the newspapers.
Meaning Luffy didn't say this as a kid to Ace and Sabo too since its implied he said the same thing as Roger
They probably said "Freest man on the sea"
My bet is we learn what they both said in Shanks' flashback since Shanks said to Rayleigh in Sabaody that "Luffy said what Roger did" and that's why he gave him the strawhat. Also shanks is the most only person who could possibly know what both Roger and Luffy said since both Roger and Luffy probably repeated this line a couple of times and only Shanks was around both of them and in a single flashback we will learn what both said