Prince George, Princess Charlotte and Prince Louis stepped out in support of their mom, Kate Middleton, at her poignant carol service.

On Dec. 6, Prince William escorted the couple’s three children to Westminster Abbey in London for Kate’s fourth annual carol service. The outing marks the first time the trio of siblings have made a joint public appearance since Trooping the Colour in June.

The event takes place amid a challenging year for the family, as Kate has faced her cancer journey. When she announced her cancer earlier this year, Kate said that she William “have been doing everything we can to process and manage this privately for the sake of our young family. As you can imagine, this has taken time.”

George, Charlotte and Louis carried notes to put on the “Kindness Tree” outside the venue, where guests were invited to dedicate a decoration to someone who has been a source of support in their lives.

“It’s been such a difficult year for them, it’s wonderful to see them all together,” Sophie Mirman, founder and creative director of Trotters, the brand behind Charlotte’s burgundy coatdress, tells PEOPLE.

In a candid moment at the event, Kate reflected on her challenging year, telling a performer, “I didn’t know this year was going to be the year that I’ve just had…the unplanned.”

The Princess of Wales, 42, spearheaded the first Together at Christmas service through The Royal Foundation in 2021 as a way to thank those across the U.K. who gave back to their communities during the COVID pandemic. The concert has become an annual tradition at Westminster Abbey ever since, and a staple on the royal family’s calendar.

Continuing tradition, Princess Kate was first to arrive at Westminster Abbey, and Prince William appeared with their children shortly after. As host, Princess Kate is known to show up first to greet volunteers and guests, before her family, who usually arrive just before the service starts.

Like last year, Prince of Wales is due to a deliver a reading at the service, which includes the poignant line, “The people who walked in darkness have seen a great light; those who lived in a land of deep darkness— on them light has shined.” William’s reading of the first Lesson is especially meaningful, following what he called a “brutal” year as his wife and father, King Charles, were both diagnosed with cancer.

Prince George, 11, Princess Charlotte, 9, and Prince Louis, 6, looked more grown up than ever at the festive event, which marked their second major royal outing of the year. The Wales family coordinated in red hues, with George and Louis twinning in red ties that matched Kate’s coat with bow by Sarah Burton for Alexander McQueen (her wedding dress designer!), while William’s tie and Charlotte’s Trotters coat (which is a royal repeat from last year) were the same burgundy shade. In a doting dad move, William placed a hand on his daughter’s back as they all made their way inside.

“It’s a timeless classic, it’s very well-cut and very comfortable,” Mirman of Trotters tells PEOPLE. “It’s handmade in a little workshop in Portugal, and literally every piece of that coat is hand-stitched. It’s quite an extraordinary piece of work. Coats are usually mass-produced, but each coat is made by one person from beginning to end, from cutting the fabric to hand-making the buttons to stitching the velvet collar.”

“Charlotte just looks absolutely gorgeous,” she adds. “It’s wonderful to see her growing up in our designs — does become very special, especially at these big occasions.”

George, Charlotte and Louis the only kids who joined the adults for the night out, either. Pregnant Princess Beatrice and her husband, Edoardo Mapelli Mozzi brought along his son, Christopher Woolf. Christopher Woolf is affectionately nicknamed Wolfie, and at age 8, is a year younger than Princess Charlotte. They walked in with Zara Tindall, who is also a mom of three with her husband, Mike Tindall.

The Prince and Princess of Wales’ children last publicly appeared together at Trooping the Colour in June for the parade celebrating their grandfather the King Charles’ official birthday as sovereign.

A month later, on July 14, Prince George and Princess Charlotte enjoyed special, one-on-one outings with their parents—Charlotte attended Wimbledon with mom Kate, while George joined dad William at the UEFA European Championship.

Last year, the Prince and Princess of Wales brought all three of their children to Kate’s Together at Christmas concert for the first time, marking Prince Louis’ debut at the event.

Both Kate and William, 42, are dedicated to ensuring their children enjoy as normal a childhood as possible. A palace insider previously told PEOPLE that William “absolutely loves” the “stabilizing normality” that Kate brings to raising their kids.

Every iteration of the Together at Christmas carol service so far has had a unique theme, and Princess Kate chose a meaningful motif after a difficult year in which she and King Charles, 77, both announced cancer diagnoses. The King’s treatment is ongoing, while the Princess of Wales shared in September that she completed chemotherapy.

Kensington Palace previously announced that the 2024 concert themes would be love and empathy, a poignant choice after a challenging time.

“This year’s service provides a moment to reflect upon the importance of love and empathy, and how much we need each other, especially in the most difficult times of our lives. The service will shine a light on individuals from all over the U.K. who have shown love, kindness and empathy towards others in their communities,” Princess Kate’s office previously said in a statement.

The palace said that 1,600 people would convene in Westminster Abbey on Dec. 6 for the holiday concert, welcoming those who have supported others in their communities through personal relationships, work or volunteering, plus others who experienced difficult times this year.

The Westminster Abbey choir, Olivia Dean, Gregory Porter, Paloma Faith and JP Cooper will perform, with Prince William, Olympian Adam Peaty OBE and actors Sophie Okonedo CBE, Michelle Dockery and Richard E. Grant due to deliver readings on the themes of love and empathy.

Princess Kate’s holiday concert is also being celebrated with 15 community carol services across the U.K. during December, and the Westminster Abbey pageantry will later be broadcast as part of the Royal Carols: Together at Christmas special airing on ITV1 and ITVX on Christmas Eve.

Guests of those festivities, as well as the banner night at Westminster Abbey, received a letter from the princess with an emotional message about love being the “greatest gift” there is.

“This Carol Service is a heartfelt celebration of every one of you, and a reminder that at Christmas, and throughout the year, we must all shine for each other,” she wrote later in the letter. “Because in times of joy and sadness, we are all each other’s light. “