Oriwa Kemp was 17 when Nia died and was jailed for assaulting the little girl.
Nia's mother Lisa Kuka was convicted of manslaughter.
Her boyfriend Wiremu Curtis and his brother Michael were jailed for life after being found guilty of Nia's murder.
The Herald revealed earlier this month that Kemp was back before the courts.
For legal reasons she could not be named.
But on Friday she admitted charges of assault, theft and breaching intensive supervision conditions.
A charge of disorder was withdrawn.
It is understood the assault occurred on Karangahape Rd in central Auckland soon after Kemp was caught stealing a bottle of Coca-Cola and can of Pringles chips.
Kemp will be sentenced next month.
In 2018 she spoke to the Herald about her troubled life, Nia and her own children - who have all been taken into care.
At the time, Kemp was pregnant with her fifth child and was battling to keep custody of that baby when he was born.
The baby was uplifted by Oranga Tamariki shortly after birth.
Kemp revealed to the Herald that she left home at 12 to live with her boyfriend Michael Curtis.
By 13 she was addicted to methamphetamine and being subjected to regular domestic violence.
She gave birth to her first child at 14 and her mother died a year later, pushing Kemp further into a spiral of drugs and destruction.
When she spoke out about her life she said she did not want sympathy.
Rather, she wanted people to know the facts before they judged her.
"I don't want anyone to feel sorry for me, no way," she said.
