UPDATED 8.21AM: The death of a three-year-old boy at the hands of his caregivers is being described as a tragedy that should never happen again.

Moko Rangitoheriri was kicked, slapped, stomped and bitten by his caregivers, and was left for up to four days with the injuries before anyone called for help.

His caregivers, 26-year-old Tania Shailer and 43-year-old David Haerewa, have pleaded guilty to manslaughter and ill treating a child.

Child Matters chief executive Anthea Simcock said the circumstances that led those adults to mistreat that child must not be allowed to reoccur.

"What can we do that we're not producing other adults who might end up doing such horrific things to children in the future."

She said there may have been people who had concerns about the situation and could have intervened

"The message is always if you have any worries about the way you think a child is being treated, it might have ended up saving a child's life."

Dame Lesley Max, chief executive of Great Potentials, told Mike Hosking she's concerned the more serious charge of murder was dropped by the Crown, but understands why it was done.

"My sense is that in going to the manslaughter charge, they got a full confession and a very detailed confession, and they got a conviction. A conviction is often much harder to get with a charge of murder."

Dame Lesley also said that child welfare reforms should focus on preventing harm at home:

"Although it was not parents who inflicted this, nevertheless I think with an emphasis now on government reforms, more on predictive risk, we might be getting a little closer, and I hope that would be the case."

Meanwhile, law professor Chris Gallavin wants the Law Commission to consider different degrees of murder for cases like this. 

"New Zealand homicide laws are a complete mess, a total basketcase," he said. "They need a review right from the ground up."