Despite her cheeky nature, Princess Charlotte had shown what a polite little girl she could be earlier on in the day.
Guests included newlyweds the Duke and Duchess of Sussex.
Prince Louis, the third child of Prince William and wife Catherine, the Duchess of Cambridge, been christened at a small, private ceremony overseen by close family but without his great-grandmother the Queen.
The 92-year-old monarch has a busy week ahead, marked by a visit from US President Donald Trump in Windsor on Friday.
Welcome (officially) to the world, little Prince Louis.
But although the ceremony might have been a snooze for a tiny prince, the fashion from the day wasn't.
Charlotte gave a cheery wave as she walked towards the cameras while George looked more hesitant.
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The 36-year-old royal wore the go-to luxury British label during previous christening services for her two older children, Princess Charlotte and Prince George. The cream gown was styled on the Queen's own wedding dress.
Photos show Prince Charles, 69, red-faced and flustered as Camilla helps him with the poppers of his gloves.
The princess and her brother Prince George held hands with their father, Prince William, the Duke of Cambridge.
Louis, born April 23, is being christened at the Chapel Royal, the same church where his older brother, Prince George, was christened.
The new robe was created by the Queen's dresser Angela Kelly.
Royal christenings are steeped in tradition, with customs passed down through the generations. Afterward, the Duke and Duchess of Cambridge hosted a tea party at Clarence House and served a portion of their wedding cake that they had saved since 2011.