Warning: Graphic content

A man who watched a small Queensland boy die in agonising pain smiled and laughed as he walked free from court.

Matthew Scown pleaded guilty to the manslaughter of Tyrell Cobb and was sentenced to four years behind bars - but a plea deal reduced the sentence and it was suspended because he had served almost three years on remand, meaning yesterday he walked away from the Supreme Court in Brisbane a free man.

There was no doubt Scown was delighted.

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In extraordinary scenes, the 34-year-old smiled for the cameras and laughed to himself as he walked away from court through a media scrum. Even when asked directly about the death of 4-year-old Tyrell, he giggled to himself.

Victim advocates say "soft" prosecutors are using plea bargains to speed up cases, but are letting child killers free.

The founder of child protection agency Bravehearts, Hetty Johnston, told the Courier Mail the downgrading of Scown's charge from murder left offenders "laughing" at the system.

"They're doing all these plea bargains just to move cases through ... [it's] expedition at the cost of these kids."

She added: "I think if the DPP had the resources and the backbone they should have gone for him."

Tyrell died on the Gold Coast in May 2009 from internal bleeding and stomach injuries caused by blunt-force trauma.

A post-mortem examination revealed he had 53 bruises and 17 abrasions from head to legs when he died.

Scown was charged with murder a day after the boy's death in 2009, but the case was dismissed at a committal hearing in 2010.

Committal hearings are held to see if there is sufficient evidence for a case to advance to trial.

Tyrell's mum Heidi Strbak, who was Scown's partner, will face a manslaughter trial next month.

Scown was not accused of causing the fatal injury to Tyrell, but of standing back and watching the tiny boy die as his condition got worse. He called emergency services only after Tyrell had begun vomiting bile.

Prosecutor Phil McCarthy said Scown told the operator "it looks like he's going to die".

Justice Martin Burns said Scown was not responsible for the injuries that killed Tyrell, but had failed in his duty of care to the boy.

The judge said Tyrell Cobb
The judge said Tyrell Cobb "would have survived if he received treatment".

The judge said he didn't know how Scown could deal with his failure to act.

"The horror of all of this is that the injuries Tyrell sustained were treatable and that he would have survived if he received treatment," he said.

Scown spent a total of two years and eight months in jail while on remand.

Queensland Premier Annastacia Palaszczuk said this morning her Attorney-General would review the case and said she was open to advice from anyone - including Bravehearts' Hetty Johnston - who had ideas about ways to improve the law.