It's not what they said that matters but the simple fact that they talked as if they knew each other well.
The mystery throughout the movie is who is doing the surveillance and why? How did they get access to family things a stranger would not know? What is the motive?
When trying to unravel the mystery, Georges visits Majid but initially doesn't even recognise him. There has been no obvious connection between the families since Majid was a child and it becomes obvious that Majid is not the perpetrator of the surveillance tapes (I won't reveal how).
Pierrot is Georges' somewhat disaffected teenage son but is too young to have known of the events relating to Majid. There is no obvious connection left to explain the mystery tapes. Until we see the conversation.
While the ending is possibly ambiguous enough to leave the mystery open, the connection between Pierrot and Majid's son is clearly meant to be significant and explaining the mystery seems like the intended effect.