Almost 140 children under the age of 14 were held in detention in Western Australia in the last financial year without ever being sentenced, whilst mental health outcomes across the state decline.

New data from the state's Commissioner for Children and Young People shows worsening outcomes for children and young people, including declining mental health across key areas such as learning, behaviour, safety and wellbeing. At the same time, the poverty rate has risen to 15.6 per cent, affecting an estimated 103,900 children and young people.

Across five key developmental domains — physical health and wellbeing, social competence, emotional maturity, language and cognitive skills, and communication skills and general knowledge — Western Australia's results have deteriorated faster than the national average.

Only 33.9 per cent of Aboriginal children and young people were developmentally on track across all five domains, nearly 20 percentage points below the state average. Aboriginal children are also almost twice as likely to be developmentally vulnerable at age five — 42.5 per cent compared with 23.5 per cent for all children.

The report calls for "targeted, culturally appropriate, and place-based supports to ensure all children have the same opportunities to grow, learn, and thrive".

Commissioner Dr Jacqueline McGowan-Jones said by missing early-warning signs a child is struggling, "we are not just missing a moment, we are setting the stage for far more serious harm".

"School exclusion, youth offending and long-term mental health challenges are the predictable outcomes of problems we could have addressed with early intervention," she said.

In the youth justice system — currently under examination through a Senate inquiry — children as young as 10 are being held in detention without being found guilty of a crime.

On an average night in Western Australia, between four and five Aboriginal boys aged 10 to 13 are detained without sentence, despite explicit recommendations from the 1991 Royal Commission into Aboriginal deaths in custody.

"The current minimum age of criminal responsibility in WA fails to align with international standards and does not prioritise the best interests of the child," the report states.

"Detention should only be used as an option of absolute last resort and emphasis must be placed on community-based programs that deter young people from the criminal justice system."

The report also highlights high reoffending rates, with more than 58 per cent of young people in Western Australia returning to detention. Whilst the rate has dropped significantly over the last decade, Aboriginal children and young people are incarcerated at almost 22 times the rate of non-Indigenous children.

The report delivers a blunt assessment of the system.

"Jailing is failing," it states. "Especially when we are talking about children, some of them as young as 10."

International scrutiny has intensified. Last year, delegates from the United Nations Working Group on Arbitrary Detention were denied access to both Unit 18 — the youth detention unit inside Perth's maximum-security Casuarina Prison — and Banksia Hill Detention Centre. Despite this, the group labelled Australia's youth justice system a "stain on Australia's reputation".

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Nationally, around one in three young people in detention is estimated to have Fetal Alcohol Spectrum Disorder, with most cases undiagnosed.

Echoing multiple submissions to the Senate inquiry, the Commissioner's report calls for expanded access to intensive family support services, increased availability of specialist allied health services to identify developmental issues earlier, and greater investment in programs for teenagers.

"These upstream actions are essential to reducing the number of children entering or returning to youth detention and to improving long-term outcomes for young people across WA," the report said.

Dr McGowan-Jones said the state must shift away from crisis responses.

"Prevention is not optional — it is the only way to turn these trends around," she said. "Our systems are designed to intervene when children are already in distress and that must change. We need to shift our investment and our attention upstream, to the earliest years of life, where the right support can alter a child's entire trajectory."