Britain's Princess Charlotte is joined by her parents and her brother Prince George as she arrives for her first day of school on Thursday, September 5. Aaron Chown/AP

Kensington Palace released this undated photo of Charlotte to mark her fourth birthday in May 2019. The photo was taken by Charlotte's mother Catherine, the Duchess of Cambridge, at their home in Norfolk, England. Charlotte is fourth in line to the British throne behind her grandfather, Prince Charles; her father, William; and her big brother, George. Duchess of Cambridge/AP

Charlotte turns to wave at journalists as she arrives with her father and her brother George to meet the newest member of their family , her brother Prince Louis, in April 2018. BEN STANSALL/AFP/Getty Images

Charlotte smiles in a handout picture provided by the Duke and Duchess of Cambridge as she prepares for her first day at nursery school in January 2018. Handout/Kensington Palace

Charlotte arrives at Berlin Tegel Airport during the royals' official visit to Germany and Poland in July 2017. Chris Jackson/Getty Images

Charlotte is pictured at home in April 2017. The photograph was taken by her mother to mark her second birthday. HRH The Duchess of Cambridge via Getty Images

Charlotte is held by her mother as her family ends an eight-day tour of Canada in October 2016. MARK LARGE/Getty Images

Catherine holds flowers for Charlotte to smell as they leave Victoria, British Columbia, in October 2016. Samir Hussein/WireImage/Getty Images

The royal family watches an entertainer blow up a balloon during a children's party in Victoria in September 2016. Chris Jackson/Getty Images

The royal family arrives in Victoria in September 2016. DON MACKINNON/AFP/Getty Images

William and Catherine attend to Charlotte during celebrations marking the Queen's 90th birthday. Samir Hussein/WireImage/Getty Images

Charlotte and her dad are photographed during a trip to the French Alps in March 2016. John Stillwell/WPA Pool/Getty Images

Charlotte plays with a stuffed dog in a photo taken by her mother in November 2015. HRH DUCHESS OF CAMBRIDGE/Getty Images

This family photo was taken after Charlotte's christening in July 2015. In the front row, from left, are William, George, Catherine, Charlotte and Queen Elizabeth II. In the back row, from left, are Catherine's father, Michael Middleton; her sister, Pippa Middleton; her brother, James Middleton; her mother, Carole Middleton; Charles; Camilla, Duchess of Cornwall; and Prince Philip. Mario Testino/Art Partner/Getty Images

William and Catherine pose with their two children at Charlotte's christening. Mario Testino/Art Partner/Getty Images

Charlotte is pushed in her pram following her christening. Chris Jackson/Getty Images

Charlotte is carried by her mother as they arrive for her christening. Mary Turner/WPA Pool/Getty Images

The royal family arrives for the christening. Matt Dunham/Getty

Charlotte is seen with her big brother for the first time in this photo released by Kensington Palace in June 2015. HRH The Duchess of Cambridge

Charlotte sleeps in a car seat as the family leaves St. Mary's Hospital in London on May 2, 2015. The newborn princess weighed 8 pounds, 3 ounces. JOHN STILLWELL/AFP/Getty Images