Harry was also in attendance at the event, alongside pregnant wife Meghan.
The duchess, who is pregnant with her first child, gave a reading from the poem Our Deepest Fear by Marianne Williamson.
The pregnant royal apparently chose the reading because, “it sums up he spirit of what the Henry van Straubenzee Fund does to empower young children," PEOPLE reports.
Harry then spoke to the congregation, highlighting the work the charity does for young girls and giving them an education.
“As my wife said many years ago when working on menstrual health and health education, this is not about periods but potential,” he said, referencing Meghan’s visit to India with World Vision in January 2017 to highlight the importance of breaking the stigma surrounding menstruation.
The 34-year-old royal dad-to-be was very close to Henry van Straubenzee, with the pair both attending Ludgrove prep school.
Prince William counts Henry's older brother, Thomas, as one of his dearest friends. Thomas is the godfather to William's second child, Princess Charlotte.
The royal couple's outing comes after Kensington Palace confirmed both the Duke and Duchess of Cambridge and Duke and Duchess of Sussex would be attending Christmas at Sandringham.
Will and Kate will stay at their nearby home of Anmer Hall with children Prince George, 5, Princess Charlotte, 3, and 7-month-old Prince Louis.
Meghan may also be joined by her mum, Doria Ragland, who has reportedly been invited to join them for Christmas by the Queen.