Princess Charlotte is "very excited" about her first day of school, her proud father said as he dropped her off for the start of a new chapter in her life.
The Duke of Cambridge walked beside them, holding Prince George's hand after driving his family to the fee-paying independent school.
William parked the family Land Rover Discovery in an internal car park before the foursome headed to a side entrance where Helen Haslem, head of the lower school, was waiting to greet them.
Charlotte grasped her mother's hand with both of hers and walked close to her as she approached Ms Haslem.
The duchess smiled and asked: "How are you? Did you have a nice summer?", and Ms Haslem bent down to shake George's hand and then Charlotte's.
William beamed and said: "First day - she's very excited."
His daughter is following in the footsteps of six-year-old brother George, who has been at the school for two years.
Charlotte wore the school's transitional uniform which spans both summer and winter - a dark dress and cardigan with a blouse, white socks and shoes.
She had her hair in a practical ponytail, which she kept adjusting, fixed with a bow.
Her mother carried her satchel which had a key-ring in the shape of a pony's head attached to it.
William and Kate, like other parents, get a slight reduction in fees for Charlotte's education this school year as she has a sibling at Thomas's Battersea - the school charges £19,287 for the first child and £18,915 for the second.
The duke and duchess took Charlotte to her class, where she is one of 21 pupils, to help her settle in and then waved her off for the start of her first day.
Kate missed George's first day in September 2017 as she was suffering with extreme morning sickness while pregnant with Prince Louis.
She looked relaxed, wearing a summer dress, while her husband wore a jacket, trousers and shirt.
In his welcome message on the school website, headmaster Simon O'Malley says: "Thomas's Battersea is a busy, thriving, purposeful school, educating 560 boys and girls between the ages of four and 13.
"As you would expect of any Thomas's school, the most important school rule is to 'Be Kind'. We offer a rich and broad curriculum, with art, ballet, drama, ICT, French, music and PE all taught by specialist teachers from a child's first day in school."
He added: "We hope that our pupils will leave this school with a strong sense of social responsibility, set on a path to become net contributors to society and to flourish as conscientious and caring citizens of the world."