George, Charlotte and Louis Volunteer at Baby Bank with Mom Kate in Private Family Outing: See Video

Kate Middleton is teaching her children about the spirit of giving during the holidays.

Prince George, 10, Princess Charlotte, 8, and Prince Louis, 5, joined their mom for an under-the-radar volunteer session sorting clothes, toys and essential goods for children and families in need at the Baby Bank in Maidenhead, Berkshire, which is near the family’s home in Windsor.

In a new video released by Kate’s office at Kensington Palace on Monday, the children look through bags of donations and select toys for other children to enjoy. At the start of the film, Kate drives up in the family's SUV and helps the children unload donations that they brought along.

The royal siblings were told that the gifts and donations were sorted by teams of people giving their time for free.

"You're the volunteers for this evening," Kate says.

A staffer then asks the royal children to "try and choose some presents for children who were similar ages to you guys. So if you think about what you would like to play with."

With that, Louis picks up a King Kong-style gorilla, saying triumphantly, "This is a big guy."

The children take care to pack the goodies and write labels on some of the presents. Charlotte holds up a onesie decorated with Welsh flag emblems. "This is Welsh," she says proudly, perhaps in a nod to her parents' titles as the Prince and Princess of Wales.

Toward the end of the outing, Kate puts her arm around George's shoulder and says, "You can see how rewarding this kind of work is knowing you are helping out others."

It is the fifth year that the Maidenhead center has hosted a holiday session — packing 1,500 gift bags in the process.

Last month, Kate brought a set of books to Sebby’s Corner, a baby bank in Barnet, north London. She has since asked for people to pitch in and donate to baby banks in the lead-up to Christmas.

Kate honored those who work in the sector by including many of them at her annual carol concert at Westminster Abbey on Friday. Bianca Sakol, the founder of Sebby's Corner, was among the congregation, seated just behind the royal family.

“The service was absolutely beautiful, absolutely perfect — we loved it,” she tells PEOPLE.

When Kate arrived with William and their children at the carol concert, the princess smiled at Sakol and her trio of colleagues from Sebby’s Corner.

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They were among a group of baby banks that had been invited (another, from Baby Basics, was represented) along with other organizations that support families and kids in the early years of life.

“It is wonderful that she invited so many different organizations,” she tells PEOPLE. “It shows how important the early years are and all the work that baby banks are doing. For the early years to be put at the forefront of this is really special.”

“To be invited shows that what we are doing is important and that she values our work. We were thrilled. To have baby banks such an important part of the service was really magical," she adds.