PRINCE Louis clutched a sweet message to his granny and grandpa thanking them "for playing games with me" at Kate's carol concert.
The note could be seen as he entered the celebrations at Westminster Abbey with Prince William, Princess Charlotte, and Prince George.
It read: "Thank you to Granny and Grandpa because they have played games with me."
Princess Charlotte and Prince George could also be seen carrying their own notes.
After the service, the children hung up their messages on a "Kindness Tree" stationed outside the abbey, where they were also joined by other youngsters.
Guests had been invited to write the name of someone who had helped them this year on the pieces of red card.
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The display was to celebrate this year's theme for the concert - love and empathy.
Explaining the theme to her 1,600 guests before the service, Kate wrote in a letter about “how much we need each other, especially in the most difficult times of our lives”.
She also said that Christmas “encourages us to turn to love, not fear. The love that we show ourselves and the love we show others.”
Louis' heartfelt note comes after the Princess, 42, shared an adorable video in September, showing the three children playing games with the Middleton grandparents, Michael and Carole, on a family holiday in Norfolk.
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It was in this video that Kate also revealed that she had completed her chemotherapy treatment.
She described her "relief" and explained how her family had navigated "stormy waters" of what she said was a "complex, scary and unpredictable" struggle.
Kate arrived at the carol service earlier this afternoon wearing a red coat designed by Sarah Burton for Alexander McQueen.
In heartwarming scenes, she and Wills were seen standing either side of their brood holding candles and singing festive Christmas carols.
A touching moment unfolded as the royal mum-of-three helped the young trio light their candles from her own.
Announcing the concert, a Palace spokesman said: "This year’s service provides a moment to reflect upon the importance of love and empathy, and how much we need each other, especially in the most difficult times of our lives.
"The service will shine a light on individuals from all over the UK who have shown love, kindness and empathy towards others in their communities.
"This year, Her Royal Highness wanted to celebrate the many people supporting those in need.
"Individuals who have inspired, counselled, comforted, and above all else shown that love is the greatest gift we can receive.
"This theme takes inspiration from the Christmas story which encourages us to consider the experiences of others and the important human need of giving and receiving empathy.”
The service, which is Kate's fourth, comes after a tough year for the royal family - after Charles and Kate were both diagnosed with cancer.
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Camilla has also been battling post viral fatigue.
The concert marks the first time the family have been seen together in public since their appearance at Trooping the Colour, the King’s birthday parade, in June.