The Duke of Cambridge was joined by Prince George and Princess Charlotte at a half-marathon on Sunday at Queen Elizabeth's Sandringham Estate

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Prince William is having some Father's Day fun with his kids! 

The Duke of Cambridge, who will celebrate his 39th birthday on Monday, was joined by son Prince George and daughter Princess Charlotte on Sunday to help kick off a half-marathon at Queen Elizabeth's Sandringham Estate.

Together the trio - who were all dressed casually for the outing - helped count down the start of the race and also cheered on all of the runners. William was also seen holding hands with George, 7, and Charlotte, 6, as they made their way to the stage. 

Missing from the day at the races were William's wife Kate Middleton as well as their youngest child, son Prince Louis, 3.

Prince William, Duke of Cambridge arrives at the Sandringham Estate in Sandringham, Norfolk to host the start of a half marathon with his kids Prince George and Princess Charlotte.
Prince William, Prince George and Princess Charlotte
| Credit: BACKGRID
Prince William, Duke of Cambridge arrives at the Sandringham Estate in Sandringham, Norfolk to host the start of a half marathon with his kids Prince George and Princess Charlotte.
Prince William with Prince George and Princess Charlotte
| Credit: BACKGRID

In honor of the June holiday, William and Kate also shared a special tribute.

A post on their official social media account showed a bulletin board pinned with the words "Happy Father's Day" alongside a never-before-seen photo of William and his kids, as well as shots of his father Prince Charles and Kate's father Michael Middleton.

The sentimental message also included a photo of William's late grandfather Prince Philip."Wishing fathers everywhere a happy Father's Day," read the caption of the post. 

Both William and Kate, 39, have been increasingly stepping out as their royal duties continue to ramp up and COVID lockdown restrictions slightly lessen in the U.K.

On June 11, he joined grandmother Queen Elizabeth alongside Kate, his father Prince Charles and Camilla, Duchess of Cornwall in holding court with global leaders at the G-7 economic summit.

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In the coming weeks, he will also be joined by his brother Prince Harry at Kensington Palace to unveil a new statue dedicated to their late mother Princess Diana on July 1, which would have been her 60th birthday. And then on July 22, Prince George celebrates his 8th birthday.