Prince William could then be heard telling Charlotte: "Well done".
As with her previous two children, Kate chose an all cream ensemble by Alexander McQueen, with a Jane Taylor fascinator, and beamed as she carried her youngest to the venue, while William held hands with George and Charlotte.
A royal source said it had been agreed some time ago that the 92-year-old queen would not attend the small, family affair and her absence was not due to any health issues.
The third child of Prince William and Kate Middleton, Prince Louis, was christened Monday afternoon at The Chapel Royal in St Jame's Palace in London.
Four boys rescued from Thai cave, much work still to do
They were discovered by a team of British divers on Monday and rescue operations have been meticulously planned all week. Cave rescue experts consider an underwater escape to be a last resort, especially with people untrained in diving.
All of the babies in the photographs posted on the Instagram account are wearing the original Honiton robe - the intricate lace and satin christening gown that was made for Queen Victoria's eldest daughter, and used for all royal baptisms until Lady Louise Windsor's in 2004. As they walked up to greet the Archbishop of Canterbury, Justin Welby, the little one was seen making some amusing faces before giving him a formal greeting.
The Prince has promised to build an exclusive library for Louis in memory of Princess Diana and the 1926 AA Milne classic is the library's first book.
Britain's Catherine, the Duchess of Cambridge, carries Prince Louis as they arrive for his christening service at the Chapel Royal, St James's Palace, London, Britain, July 9, 2018. Guy Pelly has been a longtime friend of the Kate and Will and Harry Aubrey-Fletcher went to Ludgrove School and Eton with Prince William.
Prince Louis Arthur Charles of Cambridge was born on April 23 - St. George's Day, England's national day - at St Mary's Hospital in central London weighing eight pounds and seven ounces (3.8 kilogrammes).
Mrs Carter is a schoolfriend of the Duchess, while Miss Middleton, a solicitor, is her cousin.