The children of Prince William have never met their other grandmother in the flesh as Princess Diana passed away more than 15 years after the birth of Prince George, Princess Charlotte and Prince Louis.

But George, 7, Charlotte, 5, and Louis, 2, are very aware of their late grandmother. In the documentary Diana, Our Mother: Her Life and Legacy, William said that the Princess of Wales is a constant subject of conversations with his kids.

William has made it a point for his children to know who their grandmother was that she was sometimes their bedtime story. Photos of Diana are all over their house as well so the children will be reminded of her existence. He said that it was important for his three kids to know that they have two important grandmothers in their lives.

On Mother's Day, George, Charlotte and Louis made cards of "Granny Diana." The card Charlotte created had a heartbreaking message as she wrote that her "Papa" always misses his mother.

It comes as royal experts criticized Prince Harry for constantly invoking the name of their mother, especially during his interview with Oprah Winfrey. Harry said that his mother might have anticipated his troubles with the royal family and he compared what he and Meghan Markle have been going through with Diana's struggles as a member of the royal family.

But royal expert Richard Kay said that the Mother's Day cards from William's kids were a strong reminder to the public that Diana had two sons. Prince William also doesn't bring up his mother as much as Harry because he doesn't want to "weaponize" her name.

William and Harry are expected to be reunited in person in July for the unveiling of their mother's long overdue statue at Kensington Palace. While some sources said that the two will be a united front, for the sake of their mother, a report from The Sun cited that it could be "impossible" for the brothers to be friendly for the cameras because of their ongoing rift.

Sources said that William has been deeply hurt by Harry's claims in the interview, sending their relationship "rock bottom." It will, allegedly, take a lot of work for William and Harry to patch up and be united so the reunion might not take place all.

William just turned 15 and Harry was turning 13 when they lost their mother, who tragically died in a car accident in Paris in 1997. The older prince had been Diana's confidant and ally when she separated from Prince Charles five years before she died.