Grandchildren on your knee
Vera, Chuck, and Dave
Lennon said of it in Anthology:
When I'm Sixty-Four was something Paul wrote in the Cavern days. We just stuck a few more words on it like 'grandchildren on your knee' and 'Vera, Chuck and Dave'.
It's worth noting that this song fits into the small category of McCartney numbers that Lennon grew to have particular disdain for. He famously called McCartney's bespoke ska parody Ob La Di Ob La Da "Paul's granny music shit", and said of this song in his 1980 Playboy interview:
[That was] Paul completely. I would never even dream of writing a song like that. There are some areas I never think about and that is one of them.