The girl has been named by police as Nuria Sajjad.
In a statement, her family said: “It is with profound sorrow that we announce the passing of our beloved Nuria on Sunday, 9 July 2023.
“Nuria was the light of our lives. She embodied joy, kindness and generosity and she was loved by all around her.
“We would like to thank the efforts of the emergency services, all the extraordinary staff at St George’s Hospital, the parents of Nuria’s class fellows and staff of the Study Prep for all they have done to ease Nuria’s journey.
“We request that our privacy is respected at this difficult time.”
Education Secretary Gillian Keegan said she was “shocked and saddened” at the second death.
“My thoughts are with Nuria’s family and loved ones at this truly awful time,” she wrote on Twitter.
Wimbledon MP Stephen Hammond said: “I am so saddened to hear that another young girl has lost her life.
“My thoughts and prayers are with Nuria’s family and friends as well as the staff at St George’s who have been working tirelessly and the wider school community.”
Mayor of London Sadiq Khan tweeted: “My heart goes out to the family of 8-year-old Nuria Sajjad and everyone affected by this tragic incident.”
A family statement said: “Selena was an intelligent and cheeky girl, adored and loved by everyone. The family wishes their privacy to be respected at this sad time.”
A tribute at the scene on Friday said: “Dear Selena, you will always be our shining star. We will miss you so much.”
The note included a picture of a star and had kisses on it.
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Friday: Tributes for girl killed in Wimbledon crash
A woman, aged 46, was arrested on suspicion of causing death by dangerous driving after the crash.
She was taken to hospital and her condition was assessed as non-life threatening.