Speaking to head of the lower school, Helen Haslem, Kate asked:"How are you? Did you have a nice summer?"
The teacher took a moment to bend down and shake Prince George's hand followed by Princess Charlotte's.
Miss Haslem asked the kids, "Are we ready to go? How exciting."
William then said, "First day - she's very excited."
Kate added: "George, you'll have to show her the way.
Just like with Prince George's first day, Kensington Palace shared official portraits from the big day.
"The Duke and Duchess of Cambridge are very pleased to share a photograph of Prince George and Princess Charlotte at Kensington Palace this morning."
"The photo was taken shortly before Their Royal Highnesses left for Thomas's Battersea. 📷 Kensington Palace."
Earlier this year, the Palace announced that Princess Charlotte, who was born on May 2 2015, would be embarking on her big new milestone with the following statement:
"Princess Charlotte will join Prince George at Thomas's Battersea from September 2019 this year."