A cultural shift is needed to better care for South Australian children with serious mental health issues, with thousands being let down by the state's health system, an advocacy group says.

Parents for Change founder Kate Stephens said every week children in crisis are being turned away from hospital while the state's child and adolescent mental health service has intolerably long wait times.

"Thousands of young people are being denied care at every access point," Ms Stephens said.

"In a crisis, they are being sent home with no support, no referrals and no follow-up care.

"In a first-world country, this is unacceptable."

Parents for Change will rally on the steps of parliament house in Adelaide on Saturday and will present a 12,000 signature petition calling for healthcare reforms to Opposition Leader Peter Malinauskas and SA-BEST upper house MP Connie Bonaros.

Ms Stephens said the number of children arriving at Adelaide's Women's and Children's Hospital experiencing a mental health crisis had increased dramatically and now accounted for 10 per cent of all presentations, or about 80 children each week.

Yet she said the hospital still had only 12 beds in its mental health ward.

Ms Stephens said a cultural change was urgently needed at the hospital and across the health system to ensure mental health was taken more seriously.

"It's abhorrent that children in an acute mental health crisis are told they are only attention-seeking," she said.

"They deserve better, and so do we as taxpayers with a right to a health system that listens, responds and adequately prepares to meet the needs of its citizens."

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