Selena Lau and Nuria Sajjad, 8, died when a car crashed through a fence and into a building at The Study Prep school in Wimbledon on July 6 last year.
A 46-year-old woman from Wimbledon was arrested at the scene on suspicion of causing death by dangerous driving and has since been released under investigation.
Families affected by the tragedy have enlisted solicitors in a bid to get answers, amid a call for time limits for resolution to be placed on cases involving a child’s death.
Trevor Sterling of the law firm Moore Barlow, which is representing families, told the Standard: “We're currently supporting the families with civil claims.
“Our aim is to ensure that all parallel investigations are undertaken properly, and as such, we may then contemplate formal legal proceedings.”
Speaking earlier on Monday, the father of Selena, Franky Lau, said he did not have confidence in the Met’s probe, and that he did not know how long the family would have to wait for an update.
He told TalkTV: “We still haven’t had any answers. There’s no update. We just don’t understand why it’s taking so long for an incident that everyone has seen.
“It’s still ongoing, that’s all we know. It could be 10 years, 20 years, we don’t know. I like to think I have faith in the police, but at this point, we don’t have much confidence.
“We just want answers, we want justice for our daughter. We’re very close to the other family, that’s how they feel as well.
“It’s no closure, no information or confirmation about whatever happened to our girls. We are frustrated. They’re suffering every day, like us.”
Meanwhile, Selena’s mother Jessie told the programme that her older daughter, now 12, has withdrawn from activities that she used to enjoy with her younger sister and sobs at night.
Detective Chief Superintendent Clair Kelland said investigators are working “tirelessly” to establish what happened.
She said: “This was a tragic incident, and we understand that the families want and need answers as to what happened.
“Specialist detectives are working tirelessly to establish the circumstances of that day, including analysing CCTV and examining the expert report from forensic collision investigators.
“We recognise that the time taken can cause further distress, but it is only right and fair to all involved that we carry out a thorough and extensive investigation.”