In footage of the royals' visit to the Baby Bank in Holyport, near Maidenhead, the family could be seen arriving by car with donations of new toys and clothes.

Inside the charity centre the princess told them: "Here there's lots of people who give up their time and come and help out."

Princess Charlotte, eight, responded "ooh la la" - before the Princess of Wales continued, "and you're the volunteers for this evening".

The children were given the task of choosing toys they would like and filling festive bags.

Rebecca Mistry, the baby bank's co-founder and co-chief executive officer, told the young royals: "What we would like you to do, is try and choose some presents for some children who are a similar age to you guys."

Ms Mistry said they helped with their Christmas campaign - where pyjamas, books, chocolates and toys are being donated to 1,600 children.

Prince Louis, five, could be seen holding a large toy gorilla and saying: "This is a big guy."

In another clip, Princess Charlotte was seen collecting toys while Prince George, 10, was shown carrying a festive bag bulging with gifts.

The children also helped to sort out children's pyjamas and at one point the young princess and her mother burst out laughing after pulling out a baby's bodysuit decorated with Welsh dragons.

Holding it up, Charlotte said: "This is Welsh."

The Princess of Wales was also spotted with her arm around Prince George saying: "You can see how rewarding this work is, knowing that you're helping out others", with her son replying: "Yes".

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The princess made an official visit to the baby bank in April and returned a few weeks ago with her children for the private evening event.