Concepcion Rodriguez, 26, unloaded six other children from the motor when she arrived home from a store on Wednesday afternoon but forgot the Benny Jnr.
He perished in a baby seat on the driveway outside their home in Corpus Christi, Texas, as outside temperatures reached 100F – making the heat inside the vehicle 140F.
Mrs Rodriguez only remembered about the child when her husband, 30, returned from work three hours later and asked where he was.
The found Benny unresponsive and rang an ambulance. Paramedics battled to revive the toddler but he was pronounce dead.
Police said that they had arrested Mrs Rodriguez and she has since been charged with injury to a child.
Family friend Cornelio Reyna paid tribute to the youngster as a ‘very happy kid’ and ‘always playing with mommy and daddy’.
‘I don’t know how something like this could’ve happened, I really don’t know,’ she told Kris TV.
Neighbour Robert Mendoza said: ‘Oh my God, oh my God it hurts my heart. You know I got little ones too. I got grandkids and I feel very sorry for them, very sorry for them.’
Friend Melinda Moore said the tragedy was ‘terrible’.