Princes William and Harry and Kate Middleton open up Buckingham Palace for a children's tea party

Prince William, Kate Middleton and Prince Harry hosted a garden tea party to honor an incredible group of 850 kids. The royal trio had a blast with their young special guests, who were the children of men and women who died serving their country in the armed forces. Photo: Andrew Parsons - WPA Pool/Getty Images

The Duchess of Cambridge looked as graceful as ever at the outdoor festivity, wearing one of her favorite dresses for the occasion. Designed by French fashion label Chloe, Kate’s knitted cream dress swirled with springtime elegance. This is not the first time the 35-year-old has donned the See by Chloe long sleeve dress. Kate was previously seen wearing the outfit at another children’s event during her royal tour of Canada with her family in 2016. Photo: Andrew Parsons - WPA Pool/Getty Images

Kate gleefully launched a slingshot off the steps of the palace during the event. Photo: Andrew Parsons - WPA Pool/Getty Images

At one point, Prince Harry tried his luck at plate-spinning. The 32-year-old was joined by the widow and six-year-old son of Fusilier Lee Rigby, who was killed by two Islamic extremists in 2013. The three shared a laugh as the royal struggled to keep his plate in the air. Photo: Andrew Parsons - WPA Pool/Getty Images

“We're here to celebrate you,” Prince Harry said during a moving speech, which addressed the children having to cope with their tragic losses. "And to remind you that we as a family, we as a nation, and we up and down the country will never ever forget about the sacrifices that every single one of you have made... You are all together, you are one community of people that have been through the same experiences and for that every single one of us here, all the organizers, the three of us, the whole of our family and the whole of this nation, thank you so, so much for everything you guys have done." He then added with a smile: "I can assure you that Buckingham Palace gardens have not seen this much fun, ever." Photo: Andrew Parsons - WPA Pool/Getty Images

The royal trio had a blast with the children. Seen here, Prince Harry joked around with a little Spider-Man, who clearly enjoyed the event's face painting. Photo: Andrew Parsons - WPA Pool/Getty Images

Kate took the time to meet guests one on one during the outdoor event.Photo: Getty Images

Kate posed for photos and was all smiles during the sunny day in London.Photo: Andrew Parsons - WPA Pool/Getty Images

The royal trio spoke with Samantha Davidson and her children Jayden, 8, Jamie, 6, Amelia, 4, backstage during the tea party.Photo: Andrew Parsons - WPA Pool/Getty Images

Prince Harry entertained the kids while his brother and sister-in-law spoke with Samantha, proving once again he is the best uncle!Photo: Andrew Parsons - WPA Pool/Getty Images

The Duchess managed to keep her light yellow See by Chloe dress spot-free as she checked out the arts and crafts station at the tea party.Photo: Andrew Parsons - WPA Pool/Getty Images

Kate sandwiched herself between a little girl and her stuffed animal and a ginger-haired boy during the tea party.Photo: Andrew Parsons - WPA Pool/Getty Images