Upon being greeted by 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.
Simon O'Malley, Headmaster at Thomas's Battersea echoed the family' excitement.
He said: "We are delighted that The Duke and Duchess of Cambridge have decided that Princess Charlotte will join her elder brother, Prince George, at Thomas's Battersea."
"We greatly look forward to welcoming her and all of our new pupils to the school in September."
Thomas’s Battersea, which describes itself as a Christian school, open to children of all faiths, believes in “praise as the greatest motivator”.
Parents are told their children will be in an atmosphere which seeks positive relationships between pupils, teachers and parents.
George, who is six, started at the school in September 2017.