The mother of Moko Rangitoheriri - the three-year-old killed by his caregivers - has complained to the Independent Police Conduct Authority about how the case was handled.
In June, Tania Shailer and David Haerewa were sentenced to 17 years in jail for the manslaughter of the Taupo boy last year.
A lawyer for his mother, Nicola Dally-Paki, has confirmed a complaint has been laid with the authority.
It concerns how Ms Dally-Paki was dealt with by police, and that charges were not laid in relation to the treatment of the toddler's sister.
The IPCA said the complaint had been received and was yet to be assigned to an investigator.
Shailer and Haerewa are appealing against their sentence at a hearing in the Court of Appeal on Thursday.