The New Zealand Law Society believes processes around plea bargains are robust, despite public outcry.
On Monday, Tania Shailer and David William Haerewa were each jailed for 17 years after they admitted the manslaughter of three-year-old Moko Rangitoheriri.
But Law Society president Kathryn Beck doubts their original murder charges would have made any difference.
"If you look at what the maximum penalty for manslaughter is and the penalty for murder, they're both the same, which is life. In this case you would say that the sentencing that was available to the judge still gave her significant breadth."
Attorney-General Christopher Finlayson has also spoken out about the plea bargain, saying it was necessary to ensure the pair went to jail.
Ms Ardern told Paul Henry this morning the problem isn't with plea bargains -- it's just that Shailer and Haerewa shouldn't have been offered one.
"Based on the summary of facts, it was pretty clear what happened to this child was horrific and it happened at the hands of either one of these two individuals, or both, and I'm sure the jury would have been able to find culpability there."
She said Crown prosecutors would have taken into account that the maximum penalty for manslaughter, just like murder, is life.
"We heard the argument that you can still get a life sentence with manslaughter, but a question that I raised with the Attorney-General was surely there's a public interest in calling a crime what it is.
"Who knows what they would have been convicted of, but let's put it to a jury."
The pair will be eligible to apply for parole in nine years, and Moko's mother says his killers got off lightly.
Nicola Dally-Paki told Story that the judge has let her and Moko down.
"For that judge to sit there and give out what she gave out, it's not good enough, as a mum, as Moko's mum."
Ms Dally-Paki is now fighting for custody of her other children, who were taken from her after Moko's death.
She says her other children are suffering emotionally.
"It's not just the death of Moko, it's going through this trauma for nearly a year now. So that will be my main focus -- fix my babies."
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