Kate was also seen to reassure George inside the abbey when the young prince rubbed his eye as the Waleses joined other members of the royal family and the world in a final day of mourning.
Princess Charlotte was overcome by emotion as the Queen's coffin was lowered into the hearse. Photo / AP
Their little brother Prince Louis, 4, was deemed too young to attend.
Charlotte, who is third in line to the throne, paid touching tribute to her "Gan Gan", as she called her, with a small detail in her outfit.
Charlotte wore a horseshoe brooch on her black dress, in what was a sweet nod to the Queen's well-known love of horses.
Her Majesty gifted the young princess with the brooch, which is made of diamonds.
Charlotte seen wearing the brooch outside Westminster Abbey. Photo / AP
Earlier, the young royals were pictured arriving via car to Westminster Hall with Kate and Queen Consort Camilla.
It comes after Kate Middleton made a heartbreaking admission to Australia's Governor-General ahead of the funeral, revealing how her children were handling the passing of their beloved "Gan Gan".
Governor-General David Hurley said in an interview, "The younger one [Louis] is now asking questions like, 'Do you think we can still play these games when we go to Balmoral and things like that, cause she's [the Queen] not going to be there?'" Hurley said.
Hurley also recalled Kate telling him that George was just now "realising how important his great-grandmother was" amid global grief over her death.
Britain's Kate, Princess of Wales, Prince George, Princess Charlotte and Camilla, the Queen Consort walk outside Westminster Abbey after the service. Photo / AP
Kate was on hand to comfort her daughter. Photo / AP
Archbishop of Canterbury Justin Welby said in his sermon at Westminster Abbey that "few leaders receive the outpouring of love we have seen" for the queen.
More people lined the route the hearse took from the capital to Windsor Castle, and many tossed flowers at the cortege as it passed. Millions more tuned into the funeral live, and crowds flocked to parks and public spaces across the UK to watch it on screens. Even the Google doodle turned a respectful black for the day.
As the coffin arrived at the castle, there were poignant reminders of her love of animals: A groom stood at the roadside with one of her ponies, Emma, and another member of staff held the leashes of two of her beloved corgis, Sandy and Muick.
Kate, Princess of Wales, from left, Princess Charlotte, Prince George, Camilla, the Queen Consort, Meghan, Duchess of Sussex and Princess Beatrice follow the coffin of Queen Elizabeth II. Photo / AP
During the committal ceremony in St George's Chapel on the castle grounds, Dean of Windsor David Conner praised Elizabeth for her "life of unstinting service" to the nation but also her "kindness, concern and reassuring care for her family and friends and neighbours."
Then the crown and the orb and sceptre were removed from atop the coffin and placed on the altar - separating them from the queen for the last time. Her coffin was lowered into the royal vault through an opening in the chapel's floor. Charles looked weary and emotional as mourners sang the national anthem.
