The cheeky statement earned a smile from her father, Prince William, who held Charlotte and Prince George's hands as they left the for post-christening reception party. The Queen and Prince Philip did not attend.
Royal babies are baptised in the same intricate cream lace christening gown.
The Duke and Duchess of Cambridge also announced their youngest son's godparents a few hours before the service.
Following the 40-minute traditional religious service, guests attended a private tea which included slices of christening cake, from a tier taken from the duke and duchess' wedding cake. They were joined by Prince Charles and the Duchess of Cornwall.
Prince Louis was christened today.
Prince George and Princess Charlotte were not present on the balcony to watch the flyover, however they did watch the festivities from a window of Buckingham Palace, where their nanny Maria Teresa Borrallo kept a close eye on them.
Also attending are Prince Charles and Camilla, Prince Harry and Meghan, the Duchess of Cambridge's parents, her brother and sister and Louis's six godparents, who are close friends or relatives of William and Kate.
Royal christenings are steeped in tradition, with customs passed down through the generations.
As People reported, van Custem has been a friend of William and Harry's since they were small children.
Prince William was an RAF search-and-rescue pilot, while his brother, Harry, served as an air traffic controller and helicopter pilot with the Army Air Corps - serving two tours of duty in Afghanistan.
Her outburst didn't last long, though, after she was picked up and comforted by her mother, adds Time magazine.
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