It’s been known for years by some; confined, by law, within the walls of courtrooms and inside newsrooms. But now the rest of Australia has been told that William Tyrrell, the missing little boy in the Spiderman costume, was in foster care when he disappeared close to three years ago.

That fact is now public knowledge because the New South Wales Supreme Court on Wednesday allowed a small, Facebook group called Walking Warriors 4 Missing Children to publish it. It wasn’t a major newspaper, or a commercial current affairs program. It was a 11,600-member Facebook group.

So who are these Walking Warriors ? Why have they spent months working tirelessly, quietly to expose a three-year-old child’s state-care status? Why have they been labelled by some as ‘vigilantes’?

Their national page is littered with posts relating to the disappearance, murder, abuse or assault of young children. But none are so omnipresent as William Tyrrell, who went missing from the front yard of his foster grandmother’s home in Kendall, on the New South Wales mid-north coast, on September 12, 2014.

Mamamia contacted the group, but they were unable to respond in time for publishing. And so the precise motivation for their tireless and no doubt expensive advocacy for this little boy remains unclear, beyond their stated goal: to create “a safer Australia in every community for all our children”.

With member Allanna Smith at the helm, they have spent months in a legal battle for the right to reveal that William was in the care of the Department of Family and Community Services under the Minister Pru Goward when he went missing. In January they won, when Justice Brereton ruled that releasing the information would not have a negative impact on the case. And on Wednesday they won again, when the Court of Appeal upheld that ruling.

On its page, the group advocates for a Coronial Inquest into William’s disappearance, something they now hope possible as a result of this week’s victory.

There is no suggestion that his foster family or the Department of Family of Community Services are in any way responsible for his disappearance. Police have dismissed any such allegations.

William is just one of 20,000 children in out-of-home care in New South Wales. And like him, the identity of most remains protected under The Children and Young Persons (Care and Protection) Act - a piece of legislation designed in the interests of the "safety, welfare and well-being" of young people in care.

While this legislation does not explicitly prohibit media from disclosing that a child is in care, it does if that child is involved or mentioned in Children’s Court proceedings - in which case the child's identity "must not be published or broadcast in any form that may be accessible by a person in New South Wales".

Because there was no mention of such proceedings in relation to William, Judge Brereton ruled in January that Walking Warriors disclosure of his in-care status would not be in breach of the Act.

But that isn't the only reason the identity of children like William remains protected.

As the CEO of the Association of Children's Welfare Agencies, Andrew McCallum, explained to Mamamia, media and the Department should only disclose confidential information about children in care if it's in the best interests of the child.

“[They remain anonymous] for their own self-esteem, for the fact that they might be targeted for any disadvantage that might be perceived, and also just to protect them from everyone knowing their business," he said. 

"These kids are the responsibility of the state, but they are not everyone’s property."

Yet Justice Brereton decided that the fact that the child in this case disappeared while in the parental responsibility of the Minister is "a matter of legitimate public interest", and would therefore outweigh any risk of - as FACS argued - having a "stigmatising effect on the child" and his relatives.

"The truth has to date been obscured: the public has admittedly been given to think that Julian*'s carers are his parents. There is a substantial public interest in accountability and scrutiny of the out-of-home care system, and in accuracy of reportage of the circumstances of Julian's disappearance," the Judge said.

"As such, it is insufficient to trump the right of free expression, in connection with the accountability, in a democratic free society, of public officials and agencies in respect of the welfare of children in out-of-home-care, which is a matter of considerable legitimate public interest."

What it's like when a loved one goes missing. Loren O'Keeffe shares her story. (Post continues after podcast.)

The dogged determination that got Walking Warriors to this point, to the supreme court, to this ruling, to national headlines, has seen their motives be widely questioned.

"If our passion, commitment to see William home and protect all our children has mistakenly seen us as vigilantes, we will accept that to some degree," Walking Warriors Australia wrote on its page.

"But let's see what Allanna Smith has done in leading the way as a huge step forward for all children in the care of the Minister. A precedence has been set in NSW... for all children currently and who may in the future be in the care of the Minister.

"FACS NEED TO BE ACCOUNTABLE! NO MORE HARM TO OUR MOST VULNERABLE AT RISK CHILDREN. NO MORE SILENCE."

In responding to the case, The Department of Family and Community Services told Mamamia it "was deeply saddened to hear about the disappearance of the child in 2014".

It declined to comment further due to the ongoing police investigation into William's disappearance.

*In the court document outlining Judge Brereton's ruling, the child was referred to under the pseudonym Julian.