Moko Sayviah Rangitoheriri, 3, died in August last year of horrific injuries inflicted by his carers.
In sentencing Moko's killers, Tania Shailer and David Haerewa, Justice Sarah Katz ruled that their previous convictions were "not relevant".
But she agreed to release the details to the Herald.
While Haerewa has a number of convictions for male assaults female, neither had been convicted of any charge relating to child abuse, neglect or maltreatment.
When sentencing an offender, a judge must look at several factors including the seriousness of the crime, the impact on the victim and the offender's background and personal circumstances. The judge must also take into account any aggravating factors including the "number, seriousness, date, relevance, and nature of any previous convictions of the offender and of any convictions for which the offender is being sentenced or otherwise dealt with at the same time".