The sweet footage contained clips from last week's family outing, where William, Kate and their three children, Prince George, Princess Charlotte and Prince Louis, joined other families to watch a special performance of The National Lottery's Pantoland at The Palladium.
WATCH: Kate Middleton and Prince William share incredible Christmas surprise in new video
The royals had revealed that during the performance, a team of elves were working on behalf of the Duke and Duchess of Cambridge, Hamleys and The National Lottery where they asked keyworkers' children what they'd like for Christmas.
Upon seeing the heartwarming gesture, fans were quick to applaud the royals - with some becoming very emotional! "What a wonderful video. Brought tears to my eyes. Well done to all concerned," remarked one follower.
"Beautiful, made me cry though, well done everyone x," another said, while a third person commented: "Wonderful, brought tears to my eyes."
Prince William and Kate with their kids at the pantomime
In the caption, alongside the festive clip, Prince William and Kate wrote: "A Christmas Surprise… This year has been extraordinary for many reasons, but through the hardship some of the most incredible people have emerged.
"This Christmas we want to say THANK YOU to all our key workers and their families for all they have done, and for the sacrifices they have made to keep us safe.
"As a small part in that effort we worked with @HamleysOfficial and @TNLUK to bring a bit of Christmas magic to the children of those key workers that came to the Palladium last Friday... We hope you enjoy the film."
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