Dean Phoenix, 44, knocked down toddler Jaiden Mangan at a level crossing in Wareham, Dorset, in March, last year.
He was cleared of causing death by dangerous driving on Thursday and now has been given a one-year jail term for the lesser offence of careless driving.
It has now been revealed by the Daily Mirror that Phoenix strangled his wife Naomi Phoenix, 26, in 2003.
He was given a life sentence in July, 2004, at Kingston-upon-Thames Crown Court.
“He’s killed two people in his life and that’s all he gets? F****ing hell!”
Authorities have not said when he was released from prison but he is believed to have been on licence when he knocked down and killed Jaiden.
The grandmother of Jaiden, Mandy Matzen, shouted “is that all he gets” when he was handed the year long sentence at Bournemouth Crown Court, on Friday.
She screamed: “He’s killed two people in his life and that’s all he gets? F****ing hell!”
Jaiden was crossing the road with his mum and sister when he was struck by Phoenix, who was driving a Sainsbury’s lorry, after he failed to see the red light.
The youngster, whose fourth birthday was the following day, died from his injuries.
Jaiden’s dad James Mangan stormed out of the courtroom with other members of his family when the sentence was given.
Judge Jonathan Fuller told Phoenix: “You have one conviction but it has no relevance to the matter and no bearing on the sentencing considerations I have to take into account.”
Phoenix had given evidence during the trial through a video link as he was recalled to prison following the crash.
Jaiden’s family asked why he was not in court, and having not been told the answer, are believed to have discovered he murdered his wife by doing a check online.
During the trial the court was shown footage of Phoenix “sarcastically” clapping a driver who had stopped at the lights.
He admitted he “made a mistake” when the film taken by a taxi’s dashcam caught him in his Sainsbury’s lorry behind a pensioner who had illegally parked a Vauxhall Astra just in front of the crossing.
He thought a car on the opposite side of the road had stopped to allow him through, rather than for the traffic lights.
Phoenix said: “I wasn’t expecting anyone to be on the crossing, I had no indication someone was in front of me.
“All my concentration was on looking ahead at the situation and moving forward so I could free up the traffic.
“It was an accident, I made a mistake, that’s why I pleaded guilty to careless driving.
“I never wanted to be in that position. I just went to work that morning, I didn’t go to hurt anyone.
“I came across that situation and moved forward when I shouldn’t have done.”
A victim impact statement by Jaiden’s mum Yasmin Dougan, read out in court and directed at Phoenix said: “My life will never be the same. All I have are memories because of your actions and stupidity.
“He’s my baby boy and I miss him so much. When I think of him now all I get is heartache and pain.
“You have taken the joy and happiness away from my life.”