George and Charlotte travelled to Westminster Abbey in the same car as Queen Consort Camilla, with Kate, Princess of Wales arriving with them.
They arrived at the church shortly after some of Queen Elizabeth II's grandchildren, including Princess Beatrice and Princess Eugenie.
The future King could be seen looking nervous, with his mother and sister behind him.
It comes after senior advisers were said to be urging Prince William and Kate, Princess of Wales to bring their eldest son.
Prince George, Princess Charlotte and Kate, Princess of Wales POOL
The boy who referred to the monarch as "Gan Gan" is now second-in-line to the throne and officials were reportedly keen to show the line of succession.
Also on Monday, Prince William put aside his differences with Prince Harry in an emotional show of unity behind Queen Elizabeth II's coffin.
Prince George arriving at Westminster Abbey POOL
With thousands of mourners in London to pay their respects to the late monarch, the Prince of Wales and the Duke of Sussex followed King Charles III in the walk towards Westminster Abbey. They were joined by the wider Royal Family – with the King, Princess Anne, Prince Andrew and Prince Edward taking leading roles.
The royals will join dignitaries from around the world in the more than 1,000-year-old abbey as they pay their respects to the beloved Queen Elizabeth II.
After proceedings conclude, the procession will set off for Wellington Arch at Hyde Park Corner.
The state hearse will later depart on its journey to Windsor as the parade gives a royal salute and the National Anthem is played.
King Charles and Queen Consort Camilla will then depart for Windsor by car, leading other members of the Royal Family.
A committal service will take place in Windsor Castle's historic St George's Chapel this afternoon, after which members of the Royal Family will depart the Galilee Porch and bring a close to the public ceremonial arrangements.
A private burial service will be conducted by the Dean of Windsor at 7.30pm, attended by the King and members of the Royal Family. Queen Elizabeth II will then be buried together with her beloved Prince Philip, Duke of Edinburgh, at the King George VI Memorial Chapel.