"Dear Granny Diana, Happy Mothers Day," Prince George wrote. "I love you very much and think of you always, sending lots of love."
Princess Charlotte wrote in her card with a rainbow heart, "Dear Granny Diana, I am thinking of you on Mother's Day. I love you very much. Papa is missing you. Lots of love Charlotte."
Louis also put a card together with a heart and his name written in blue letters for his grandmother.
Writing cards to Princess Diana on Mother's Day is a tradition that William's kids take part in each year, the Duke and Duchess of Cambridge wrote in a tweet, which also highlighted the challenges that many across the U.K. may face while being apart from loved ones amid the coronavirus pandemic.
"Looking forward to a time when we can give our mother a hug again," they wrote. "Whatever your circumstances, we are thinking of you this Mother's Day."
The couple also shared several sweet throwback images of Kate and her mother, Carol Middleton, Prince William and Harry with Princess Diana from when they were younger and photos of Kate and Prince George, Prince Louis and Princess Charlotte.
For Prince Harry, the Duke of Sussex celebrated his mother by requesting flowers to be arranged at his mother's grave as well, a spokesperson told ABC News.