The President of the WA Children's Court has warned the state's government it could be found in contempt of court for continuing to detain children in an adult prison, a claim Premier Mark McGowan has rejected.

Justice Hylton Quail described the detention of children in Unit 18 at Casuarina maximum security adult prison as "barbaric", "cruel" and "a form of child abuse".

He made the comments while sentencing two boys aged 16 and 17 for a series of burglaries and robberies committed earlier this year.

The teenagers were being held in Unit 18, established in July as a "temporary" unit for some Banksia Hill detainees due to disturbances at WA's only juvenile detention centre over a prolonged period.

The court heard that the boys were subjected to repeated lockdowns in Banksia Hill, before being moved to Unit 18.

Judge Quail noted earlier this year the Supreme Court judge had found the use of lockdowns on a 14 year old prisoner in Banksia Hill to be unlawful.

He said those "unlawful lockdowns" had continued in Unit 18. The court heard detainees in Unit 18 were often shackled when taken out of their cells.

The judge has previously alluded to judges being reluctant to hand down custodial sentences to youth because of the conditions at Banksia Hill and reiterated that the same hesitation applies due to Unit 18.

Mr McGowan claimed in response that Unit 18 remained necessary and appropriate, and said the community may be "justifiably aggrieved" that penalties handed down by the Children's Court were not seen as proportionate to the crimes.

"I don't agree with that and they shouldn't be doing it," Mr McGowan said on Wednesday.

The premier said juvenile detainees were not sentenced incarceration in Unit 18 but were sent there depending on their conduct at Banksia Hill.

Judge Quail said that in Unit 18 there is no access to education, recreation, or visits.

"There is very little human interaction, the cells are bare, with a mattress on the floor... Unit 18 is not a suitable place to detain young people," he said.

"The conditions are cruel and arbitrarily punishing and clearly have no rehabilitative effect.

"The depravation and solitary confinement is a form of child abuse inflicted on vulnerable and severely damaged children by the executive and the government, which causes short term and long term damage."

In August questions in WA parliament revealed an alarmingly high rate of self-harm and suicide attempts in both Banksia Hill and Unit 18.

WA Law Society President Rebecca Lee backed Judge Quail's view that the facility at Casuarina is not appropriate for children or young people.

"The system is broken if juvenile detainees are being housed in conditions described as cruel and punishing, and as having no rehabilitative effect," she told the ABC.

"The WA government needs to urgently look at how it can redirect funding to the programs that work to reduce the root causes of crime before behaviour escalates, and how to house children appropriately when either bail is inappropriate or a custodial sentence is to be imposed."