George, 9, arrived at Westminster Abbey wearing a navy suit with a black tie, while his sister Princess Charlotte, 7, and mother Kate Middleton were both dressed in black along with most of the members of the congregation.
Prince George's outfit may have been selected to match his father Prince William's Royal Air Force (RAF) uniform. For the service, the new Prince of Wales wore his military uniform, which included a Garter sash with RAF Pilot Wings and the Garter Star Chest Order.
Prince William's uniform was also adorned with the Queen's Gold, Diamond and Platinum Jubilee medals.
Karwai Tang/WireImage Prince George and Princess Charlotte
The Queen's funeral also marked the first time Charlotte wore a significant piece of jewelry.
Charlotte accessorized her black attire with a diamond horseshoe brooch pinned to the left side of her black coat to honor her late great-grandmother's love of horses. The sentimental brooch was gifted to the young princess by Queen Elizabeth herself, PEOPLE exclusively revealed.
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Getty Prince William and Kate Middleton take their seats with son Prince George and daughter Princess Charlotte at Queen Elizabeth II's state funeral
Princess Charlotte also wore a hat for the first time at the Westminster Abbey service, a custom for British women when attending formal events.
George and Charlotte took part in the royal family procession behind the Queen's coffin alongside Prince William and Kate Middleton as the family made their way down the aisle of Westminster Abbey. Kate, 40, escorted the little ones into the service, though William, 40, arrived earlier alongside King Charles III.
Prince William and Kate spent last week carefully considering whether to include their eldest two children in the procession and service, PEOPLE understands.
Prince George and Princess Charlotte are several years younger than William was when he attended his mother Princess Diana's funeral in 1997 at the age of 15. Prince Louis, 4, was likely deemed too young to join the rest of the family for Monday's funeral.