"I couldn't believe, in the sense of, finally today is the day that they have got somebody for Arthur," Julie Szabo said outside Surry Hills police station in inner Sydney on Friday.
NSW Police allege Gregory John Walker escalated a neighbourhood dispute by recklessly throwing a petrol bomb into a Waterloo home where Arthur and other children were sleeping on April 9, 1998.
Walker, 55, has been charged with murder and arson after being extradited to NSW following his arrest in Brisbane on Thursday.
The arrest came after homicide detectives re-opened the case in 2020.
There were public appeals for information and a $1 million reward was offered.
The arrest was not the result of a single tip-off, but Detective Superintendent Danny Doherty said the reward helped bring new information to the attention of investigators.
"It's given an incentive for people to open up and not keep a secret because, again, they've known about this for some time, and we're just pleased that they have come forward," he said.
It was also rewarding for the investigator who was able to call Ms Szabo on Wednesday to tell her a suspect had been identified.
"We don't always get to provide good news," Det Supt Doherty said.
Arthur suffered severe burns in the fire after being trapped on the top floor of the home and died weeks later in hospital.
Further details of the police investigation over the past 24 years will be presented as the case moves through the courts, which Ms Szabo acknowledges will extend an already long wait for justice.
"It's only the beginning, but I just take each day as it comes," she said.
On Friday afternoon, Walker did not appear when his case was briefly mentioned at Downing Centre Local Court where magistrate Robert Williams adjourned it for directions to October 13.
Walker's lawyer Greg Goold did not apply for bail and it was formally refused.
Outside court, Mr Goold said he had no comment given he had just received the charges and the fact sheets in the matter.
"We'll deal with those as quickly as we can and hopefully have an answer for his plea in due course," he told reporters.