One of the families, from Oregon, consisted of four kids and their father Jared.
Asked what they wanted for Christmas, the kids requested a piano, a Nintendo Switch, and a Barbie doll (the youngest, at just 2 years old, would be "happy with anything").
"Have a merry Christmas, have a wonderful Christmas," Jill Biden told them afterwards.
"And by the way, you guys have to be in bed by 9pm, you know, and asleep between nine and twelve, or he doesn't show up," the President added.
"I hope you have a wonderful Christmas."
"I hope you guys have a wonderful Christmas as well," said the father, Jared.
Then he added a sly dig at President, which we'll explain momentarily: "Let's go Brandon."
"Let's go Brandon, I agree," Joe Biden replied, apparently not getting the joke. He then asked where the family lived, but the call had already ended.
"Let's go Brandon" has become code, among American conservatives, for the more vulgar phrase "F*** Joe Biden". It's a running joke on social media, has become a staple at anti-Biden protests and has even been adopted by some Republican Party politicians.
Its origins lie in a Nascar race in Alabama on October 2. A driver named Brandon Brown was being interviewed by NBC News when the crowd behind him started chanting.
The reporter interviewing Brown suggested they were saying "Let's go Brandon," when in fact they were chanting "F*** Joe Biden".
(Alabama is not friendly territory for Biden, a Democrat. Donald Trump won the state by a whopping 62-37 per cent margin in last year's presidential election.)
US presidents' Christmas Eve events usually go off without a hitch – but not always.
Three years ago, Biden's predecessor Donald Trump committed something of a faux pas when he asked a 7-year-old whether he "still" believed in Santa Claus.
"Are you still a believer in Santa?" Trump asked.
"Because at 7, it's marginal, right?"
