The Duke and Duchess of Cambridge will be joined by Prince Harry and Meghan Markle, the Duke and Duchess of Sussex, and Prince Charles and Camilla, the Duchess of Cornwall, at the christening. Older brother Prince George walked in holding Prince William's other hand, while Duchess Kate carried their baby brother in his heirloom lace christening gown.
The Queen, 92, has a busy week ahead, with high-profile celebrations in central London marking the centenary of the RAF on Tuesday, and a visit by US President Donald Trump in Windsor on Friday. William and Kate were to host a private tea afterwards.
And little Charlotte couldn't resist sticking her tongue out at crowds while stood alongside her brother - who also pulled a face or two.
"This new version was commissioned by The Queen and made by Angela Kelly and her team as a copy of the famous original which was deemed too fragile to remain in use".
Mr. Nicholas van Cutsem is one of William and Harry's oldest friends.
Harry Aubrey-Fletcher: He went to Ludgrove School and Eton with William and his father is the Lord Lieutenant of Buckinghamshire.
Princess Charlotte has a reputation for being confident in front of the cameras that follow her and her family everywhere.
While it was widely reported that the 36-year-old recycled the look from her daughter's christening at Prince Harry and Meghan's wedding in May, eagle-eyed fans noted subtle differences.
What a magnificent (and elegant) way to welcome tiny Prince Louis to the faith and the royal family.
Welby, who will baptise the baby by pouring holy water from the River Jordan over his head, said the event would be "very nerve-wracking" but "a great privilege".
The Choir of Her Majesty's Chapel Royal performed This is the day which the Lord hath made, which was composed for The Duke and Duchess of Cambridge's wedding in 2011, and Suo Gan, a traditional Welsh lullaby.