"He threw it in the air, and when he threw it, it landed in her hood, which was really flammable," said Mrs Harawira.
Mrs Harawira quickly took Elledy to the shower and removed her clothes, running cold water on her for about 10 minutes.
Elledy received burns to her face and hand, as well as having her hair and eye brows singed.
Mrs Harawira said it was lucky her daughter did not need skin grafts.
"It was very important that I did my first aid course," she said. "A lot of people probably wouldn't have known what to do and would have gone into shock."
Mrs Harawira said her son felt guilty about what happened to Elledy, even though it was a "complete accident".
"It's really hard on him, he feels responsible."
Mrs Harawira said she was now "100 percent put off by fireworks" and warned parents to think twice about having fireworks around young kids.
She said families should stick to public displays, which were "safer and more exciting".
"Don't do it at home," she said. "Go to a show, it's more enjoyable."
It is the second serious injury already, with Guy Fawkes night not until Tuesday.
A 16-year-old suffered severe burns to his face and upper body when ingredients he was using to make fireworks ignited in the kitchen of his Auckland home on Thursday.
Police said the teenager was using gunpowder from purchased fireworks alongside other household ingredients to make his own smoke bomb using instructions he found on the internet.
Anyone purchasing fireworks must be 18 or older.
