Princess Charlotte, 10, is adorable in surprise piano duet with mom

Princess Kate was joined by a special surprise guest during a holiday piano duet that aired on British television on Christmas Eve.

Princess Kate's 10-year-old daughter, Princess Charlotte, played the piano at her mother's side in a pre-recorded portion of the televised concert at Westminster Abbey. The royal duo performed "Holm Sound" by Erland Cooper during "Royal Carols: Together at Christmas."

The opening shot featured the serenely smiling Kate's left hand at the piano — effortlessly moving on the keys even with her famed 12-carat sapphire engagement ring — with her right hand draped around the beaming Princess Charlotte on the piano bench.

"At its heart, Christmas speaks of love taking full bloom in the simplest, most human ways," Princess Kate said in the voiceover of the concert clip, posted on The Prince and Princess of Wales YouTube page. "Not in sentimental or grand gestures, but gentle ones, a moment of listening, a word of comfort, a friendly conversation, a helping hand. Presence."

Prince William and the rest of the family, Prince George and Prince Louis, appeared in scenes adorning a public Christmas tree. The royal couple released their annual Christmas card on Dec. 18, posing alongside their children in a green meadow dotted with daffodils.

"The care and compassion you give, often quietly and unspoken and without any expectation or recognition, make an extraordinary difference to the lives of others," Princess Kate said in the concert voiceover, as electric candles shone on the Christmas tree. "As the year draws to a close, I hope this season brings you moments of peace and clarity and that you, too, feel surrounded by the same love and care that you offer so freely."

The only-in-England concert featured Kate Winslet giving a personal reading about "the spirit of love" in the echoing Westminster Abbey and Chiwetel Ejiofor reading " 'Twas the Night Before Christmas."