In the clip, the pair can be seen sitting together at the piano to play Holm Sound, a piece it is understood, that they know well and have enjoyed playing together at home.
The Princess of Wales used only her left hand, while Charlotte played with just her right in Windsor Castle’s Inner Hall.
As they performed, footage was shown of guests arriving for the carol service – with the Prince of Wales and Catherine watching as their children tied paper chains bearing their names on a “Connection Tree” outside the abbey.
This is not the first time Catherine – who has passed her grade three piano and grade five theory – has played at a carol service.
At the inaugural event in 2021, she surprised audiences by accompanying singer-songwriter Tom Walker on piano as he sang his Christmas song, For Those Who Can’t Be Here.
Organisers said this year’s theme – to celebrate the “power of love and togetherness” in recognition of people across the country who contribute to society – was picked because people will be able to celebrate love in all forms in a world that can often feel fragmented and disconnected.
It is understood that Catherine wanted to include the special performance to highlight the importance of connections.
“The time, care and compassion you give, often quietly and unspoken, and without any expectation or recognition, make an extraordinary difference to the lives of others,” Catherine wrote in a letter that accompanied every order of service.