This Christmas season, Prince Louis is giving thanks.

At Friday’s fourth annual Together at Christmas carol service, Prince William and Kate Middleton’s youngest boy wrote a heartfelt note to his grandparents.

The 6-year-old left a note on the “Kindness Tree” outside Westminster Abbey in London, the location of the Christmas event, while attending the celebration with brothers Prince George, 11, and Princess Charlotte, 9.

He wrote on the message, “Thank you to Granny and Grandpa because they have played games with me,” referring to Michael Middleton and Carole Middleton, his maternal grandparents.

The 42-year-old Princess of Wales first revealed that she was receiving treatment for cancer in March, and on September 9, she revealed in a private video that she had finished chemotherapy. Kate’s parents are seen playing games at a table with Louis, Prince George, 11, Princess Charlotte, 9, Prince William, 42, and Kate in the video, which was posted on the official Kensington Royal social media pages.

“Her parents are an enduring factor in the upbringing of their grandchildren,” a royal insider told PEOPLE in March. According to reports, the Middletons occasionally join Kate on school drop-offs and host sleepovers with the three siblings.

James Middleton and Pippa Middleton, their other children, joined Carole, 69, and Michael, 75, during their daughter’s carol service on Friday. King Charles and Queen Camilla were not there, but all the Middletons were there to cheer on the Princess of Wales.

The Times reporter Kate Mansey heard Princess Kate talking to singer-songwriter and actor Paloma Faith, revealing another adorable Louis moment. Kate mentioned that Louis sadly found it difficult to hide the surprise from his sister Charlotte, who takes ballet classes, while they were talking about the ballet performance, which was a surprise that was revealed after the service.

“This morning, he said, ‘Mummy, please can I tell her – I’m bursting.’ ” the royal holiday host revealed.

At one point during the service, Charlotte was reportedly spotted standing like a dancer, her feet in third place.

According to the palace, 1,600 people will gather in Westminster Abbey on December 6 for the holiday performance, which will welcome those who have helped others in their communities through volunteer work, personal relationships, or employment, as well as those who have gone through challenging circumstances this year.