They were served slices of cake taken from a tier of William and Catherine's wedding cake, the Palace said.
As well as Prince Louis' new godparents, the couple's family - including the Prince of Wales, the newly-married Duke and Duchess of Sussex, and the Middleton family will attend.
George, four, and three-year-old Charlotte held William's hand, as a sleeping Louis was carried into the chapel by Kate.
The following year, Prince William missed the Royal Family's traditional Christmas lunch to attend the memorial service of Mr Pelly's father John, who owned two Toyota-Subaru garages in Kent.
Archbishop of CanterburyJustin Welby, the spiritual leader of the world's Anglicans, lead the service in the Chapel Royal at St James's Palace. He was born April 23, 2018. His Royal Highness Prince Louis of Cambridge is fifth in line to the throne, and the sixth great-grandchild of Queen Elizabeth.
One person wrote on social media: "Princess Charlotte walked past photographers on the way to the christening and told them "you're not coming" the sass is strong in this one".
Click through the slideshow to see who has been named a godparent to Prince Louis...