A woman, 59, died at the scene of the crash in Wantirna South in Melbourne's east on Thursday,
Victoria Police have not yet revealed the connection between the three victims, but they are believed to be related.
A Toyota Yaris, driven by a 91-year-old woman, mounted a footpath, went through a fence and hit a bench.
The driver was treated for minor injuries in hospital and was still to be interviewed by detectives as of Friday morning.
Superintendent Justin Goldsmith said the adults and child were walking in the same direction as the car was travelling when it struck them.
"It came from behind," Supt Goldsmith told reporters.
"It looks like it's (lost control) about 40m or 50m before the collision with the people who were walking on the other side of that road."
The out-of-control car continued down the road for another 200m, hitting a street sign and smashing through a fence before coming to a stop near a playground.
"No one was hit at the park, thankfully," Supt Goldsmith said.
The driver, who was "terribly shaken", had minor scratches and was taken to hospital for assessment and blood testing.
Police will look at whether speed was a contributing factor.
"That will be subject to the investigation, but it is downhill section of road, so if there has been a lack of control to some degree there is a possibility the car would have picked up speed as it's driven further down Coleman Rd," Supt Goldsmith said.
The crash happened during the first week of Victorian school holidays, bringing the state's road toll to 14 in the last seven days.
"We're facing a horrific month for road trauma," Supt Goldsmith said.
The state has recorded a number of deadly crashes involving out-of-control vehicles in recent years.
In November, a kindergarten worker was killed and a three-year-old boy was injured when a runaway truck smashed through the gate of Macedon Ranges preschool's playground.
Two weeks earlier, an 11-year-old boy was killed and four other students injured when an SUV crashed through a fence at Auburn South Primary School in Melbourne's east.
Five people were killed and six injured in November 2023 when a diabetic driver passed out behind the wheel and crashed into patrons seated outside the Royal Daylesford Hotel.
The driver was charged but the allegations were ultimately struck out after a magistrate found there was not enough evidence to support a conviction.
In 2024, the Royal Australian College of General Practitioners called for Victoria to introduce age-based regulations for drivers in line with other jurisdictions like NSW, Queensland, ACT, Tasmania and the Northern Territory, which require drivers over 75 to have a health check.
The age is 80 in Western Australia, while older drivers in South Australia must complete self-assessments.
At the time, the Victorian government rejected the call, saying drivers aged 75 already had to renew their licence every three years, which could include various tests.
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