William, 34, shared with DJ AJ King at the event that William and Kate's son, Prince George, 3, is so excited about Christmas that he has been unwrapping presents under the Christmas tree.
"The Duke said they were all really excited about Christmas and that George is already opening his presents," DJ AJ King said. "He thought it was quite funny. That's kids for you, even the ones that are princes."
Kate, also 34, wore a festive red and green dress by designer Vanessa Seward, who is a favorite of Hollywood and has worked previously at Chanel and Tom Ford.
Kate participated in icing a gingerbread house at the party, while also joining William and Harry, 32, in making Christmas decorations.
The royal trio presented the award for The Mix's volunteer of the year. They also heard from staff members who participate in online and telephone counseling at The Mix.
William, Kate and Harry have made mental health a prime component of their charitable work and launched the Heads Together campaign last spring to encourage young people to ask for help when life's challenges seem overwhelming. The campaign seeks to raise awareness and de-stigmatize mental health.
William and Kate will spend Christmas with their young children, Prince George and Princess Charlotte, in Bucklebury with Kate's parents and siblings instead of at Sandringham with Queen Elizabeth II. Harry is expected to celebrate Christmas with the royal family at Sandringham.