The mistake sent carbon monoxide, an odorless, colorless gas, flooding through their home as she got 13-month-old Parker ready for bed.
The car was still on after Petot left the garage.
"My son woke up around 12:30 a.m. and was screaming," Petot recalls.
She got out of bed to pick him up.
Petot thinks her son, Parker, may have had a headache because she now knows the level of carbon monoxide at the time was high enough to have killed them within about 20 minutes.
"Once I got dizzy, I knew I needed to get out of there," Petot said. "And walked down the stairs, opened the garage door and saw that the taillight was on."
A WSB-TV investigation has tracked more than two dozen injuries and deaths around the country connected to cars with keyless ignitions being left on, with families left wondering how this could happen.
Cars with keyless ignition have no key and are designed to start with the push of a button. But it is also easier to forget to turn off the car.
The family of Bill Thomason and Eugenia (Woo) Thomason say the couple likely never realized their mistake. Their Toyota Avalon ran inside their closed garage for 32 hours as they slept.
"We know that they went to bed that night and didn't wake up the next morning," said Will Thomason, who now lives in Atlanta.
His brother Dave Thomason also lives in the metro area, and they both rushed to Greenville, South Carolina, to get to their parents, but it was too late.
"By the time they were found they were essentially brain dead," said Will Thomason. "You can't prepare for something like this."
The sons say the active retirees had just renewed their wedding vows after 50 years and adored their five grandchildren, who they won't get to see grow up.
"Oh, it's been just absolutely terrible," said Dave Thomason. "We all know that people can get killed in car accidents due to different things, but a car sitting alone, basically doing nothing but running?"
The brothers said their pain is worsened by the number of times they've now heard the same story, with reported deaths and injuries connected to running cars around the country.
The Thomason family has filed a lawsuit against Toyota, which has already settled with several of the other families.
"Hell yeah, that makes me angry. I mean, we've lost our parents," said Will Thomason.
"Nobody is in the car, it's been running for however long. The car should have an automatic cutoff. I mean, to me that's a very easy fix," said Dave Thomason.
Records show since 2011 the federal government has been studying the need for an external alert to be placed on cars that have button ignitions, but has yet to require car companies to do anything to include an external alert.
"There's probably 25 other things that car makers do ... for safety. Well, this is a life and death safety thing and it seems to me that this is an easy thing for them to address, and they aren't addressing it," Will Thomason said.
WSB-TV tested more than a dozen of the most popular cars to see what happens when you leave them running and walk away with the key fob.
Most of the cars had a dashboard display that notes that the key fob has left the vehicle. Some even emit a low interior sound, similar to the one that reminds drivers to fasten their seat belts.
However, if a driver has left the vehicle, he or she wouldn't see that display or hear that warning. Very few of the cars made an exterior noise.
The loudest warning came from the Chevy Impala, which utilizes the car's horn.
Petot didn't hear the three low beeps her car made and she's lived with the guilt ever since.
"I absolutely take responsibility for what happened," she said. "And I think that it could happen to anybody."
But she said the price for being distracted or forgetful should not be death.
"We were incredibly lucky. We absolutely wouldn't be here," Petot said while watching Parker play in their new Marietta home. "He is definitely my little hero Valentine."
Petot said the day they moved in to their new home she purchased carbon monoxide detectors for each of the rooms.