Louis, Charlotte and George receive special message on milestone anniversary
This marks the latest chapter in the royal siblings' relationship with the charity
Prince Louis
Louis appears muted while siblings George and Charlotte join in with carols during service
The prince started the show holding his service book, but eventually had his hand in his pocket
Royal fans adore 'the sweetest photo of the Princess of Wales and Prince Louis'
The photo was taken at the Christmas Carol Service on December 5
