The couple and their two young daughters had spent the weekend house-hunting in central Victoria after deciding on a tree-change.
But as the family drove back to Melbourne on the Calder Freeway on December 6, 2021, a 43-tonne semitrailer crashed into the back of their Volkswagen.
Two-year-old Harriett, seated in the back seat, was killed instantly.
"She had the world in front of her," Ms Stewart told the County Court.
"She was such a kind soul and brought so much joy to her parents, her sister and her wider family.
"It's terribly unfair that her life was cut so terribly short."
The driver of the semitrailer, Matthew John Livingston, on Monday pleaded guilty to culpable driving causing Harriett's death.
He also admitted negligent driving causing serious injury to Mr Peckitt and Ms Stewart, as well as endangering 10 other people in the vicinity of the crash.
The court was told in the 400m before the crash site, there were several signs telling drivers to slow to 40km/h because of roadworks.
Livingston didn't hit the brakes until 0.91 seconds before the collision, striking the almost stationary Volkswagen at 86km/h.
The semitrailer then veered to the right, hitting a Toyota Hilux which rolled and trapped another driver for about two hours.
Three other cars were struck either by the Volkswagen or the Hilux, while several other motorists were forced to move their cars off the road to avoid being hit.
Ms Stewart had to be airlifted to hospital where she spent several months undergoing treatment for a catastrophic brain injury and other fractures.
She had post-traumatic amnesia for weeks after the crash and doctors advised her family not to tell her about Harriett's death until the end of January 2022.
Mr Peckitt also suffered skull fractures and bleeding in the brain, with the injuries continuing to affect him to this day.
"I woke up in the ICU in a state of delirium, completely unaware of the pain and suffering to come," he told the court.
"It's a sad reflection that the first funeral I attended was that of my daughter."
Ms Stewart also spoke of her youngest child Jemima, who at 12-months-old was trapped next to her dead sister with her two parents unconscious in the front seat.
"Jemima will never know what it's like to have an older sister ... she will not have her best mate beside her," Ms Stewart read to the court.
"She will carry this burden with her for the rest of her days."
Prosecutor Grant Hayward told the court Livingston had been sleep deprived, with other motorists seeing him swerve between lanes in the lead-up to the crash.
But the defence is disputing that the 44-year-old was fatigued before the collision.
The plea hearing before Judge Michael Tinney continues.
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