The Swiss girl, named by daily 20 Minutes as Cheyenne, was holidaying with her mother and siblings at a campsite in Muzzano, in the canton of Ticino, last July when the tragedy happened.
Left behind in the family car in scorching temperatures, the girl – initially reported as five years old – was found dead by her sisters some three hours later.
An autopsy found that she died from severe heatstroke.
Temperatures in that area on the afternoon of July 21st reached 33 degrees, reported Le Tribune de Genève.
After investigating the mother for manslaughter by negligence, on Tuesday the public prosecutor's office said it would not prosecute her.
In a statement, it said the mother was guilty of negligence, but that she had not intentionally left her daughter in the vehicle, thinking she was playing safely in the campsite.
It is “beyond doubt that the mother of the dead child... has been hit hard by the consequences of her negligence... and will continue to suffer greatly with this unforgettable burden,” said the statement.
As such, the prosecutor said the woman would not be punished further, quoting Article 54 of the Swiss criminal code which states that an offender need not be punished if he “has been so seriously affected by the immediate consequences of his act that a penalty would be inappropriate”.
Emergency psychiatrist Nadine Maetzler, who worked with the dead girl's family after the tragedy, told newspaper 20 Minutes at the time that they were overcome with grief, shame and guilt.
“The fact that children are involved makes the situation even more terrible,” she said.