In a clip, the Duke can be seen holding his kids by the hand.
And it was no different when she arrived all smiles to meet her new baby brother in hospital. Once in the building Prince William steps out of the way and Prince George, now more relaxed with his back to all the cameras, puts his arm around little sister Charlotte.
Having had a baby, and probably not much sleep - she was in labour by dawn - she looked just as serene and together as she did on the same steps after the birth of George five years ago and of Charlotte three years ago.
"Favourite thing today was this blink-and-you'll miss it moment inside the hospital where George appears to flick Charlotte's head!"
Their parents followed shortly after, but not before the crowd of photographers snapped a few photos of the new royal baby.More news: BMW unveils new all-electric SUV at Auto China show
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Princess Charlotte proved that she had already mastered the royal wave as she addressed the waiting press.
"Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs", Kensington Palace said in a statement.
A few hours later at 11:01, the Duchess gave birth to a healthy baby boy weighing 8lbs 7oz.
The Duke of Cambridge left the London hospital about 4pm local time, smiling and saying "Back in a minute". Her mother bought the clothes from the children's boutique in the Spanish northern city of San Sebastian.
The royals are yet to announce the name of their third child, however Arthur, James and Louis are all in the running.