Princess Charlotte, who usually keeps both her brothers in line, couldn’t help but laugh at Louis.
The five-year-old has entertained the nation time after time with his amusing facial expressions and tantrums.
As they walked into the Abbey’s forecourt the royal children paused for a moment to post Christmas cards, in a special postbox, containing messages for children who might be struggling this Christmas, something open to all youngsters attending.
This year the service reflects the princess’ early years Shaping Us campaign launched in January, which aims to highlight and promote the significance of the formative years of a child’s life.
It was billed as a moment to thank all those who work to support babies, young children and families in communities across the UK and among the 1,500 guests were midwives and nursery teachers.
Among the highlights of the service will be songwriter and multi-instrumentalist Jacob Collier playing last Christmas on John Lennon’s piano, which was owned by Wham! singer and solo-star George Michael and loaned by his estate.
Broadcaster Roman Kemp, the son of Spandau Ballet bassist Martin Kemp and former Wham! backing singer Shirlie Kemp, will introduce Collier and talk about the role George Michael played in his life.
Oscar winning star Jim Broadbent will read an extract from Letters From Father Christmas by Lord of the Rings author JRR Tolkien and actor Michael Ward is due to recount the moment shepherds were told by an angel about the birth of Christ and they visited Mary and Joseph, in a reading from Luke’s Gospel.
Arriving guests were treated to carols and Christmas songs performed by the Action for Children Young Carers Aloud choir, made up of 12 young carers including one aged just six.
Kate crouched down to chat to seven-year-old Oscar Burrow, from Lancaster, who when aged six climbed 12 mountains raising more than £40,000 for Derian House Children’s Hospice in Chorley, Lancashire.
His mother Kimberley Burrow said afterwards: “She said she’d heard about what he’s done and he’s doing a fantastic job, and hopes what he’s doing will inspire other children to do amazing things.”
The service began with the popular carol, Once In Royal David’s City, and the congregation also heard a duet by Beverley Knight and Adam Lambert who performed The Christmas Song, made famous by Nat King Cole.
William read the passage from the Gospel according to Luke that describes how Joseph and Mary travelled to Bethlehem to be registered, and the birth of Jesus.
The Abbey echoed with the voices of its choir and the congregation singing a selection of carols including Silent Night, Holy Night and Joy To The World, as well as Hark! The Herald Angels Sing.