An 11-year-old disabled girl who was held alone in police custody for a total of 60 hours without an appropriate adult endured "a nightmare", her mother has said.
The child was arrested three times and detained under the Mental Health Act once between February 2 and March 2 2012 by Sussex Police.
Officers restrained her using a mesh "spit hood", handcuffs and leg straps but failed to record why they had used force, watchdog the Independent Police Complaints Commission (IPCC) found.
They also failed to provide an appropriate adult to accompany her on the four occasions she was held in a police cell.
IPCC investigators found that 11 officers had a case to answer over their contact with the girl, identified only as Child H.
Her mother, known as Ms H, said in a statement through her solicitors: "My daughter's contact with the police in 2012 was nothing short of a nightmare for both of us. At the time her disability meant that she could behave in very challenging ways, but what she needed was patience, respect and the support of her mother. Instead she was locked up in a police station without me or anyone else who knew her for support.
"I know that some of the officers were doing their best, but I cannot understand why others thought it was appropriate to put an 11-year-old girl in handcuffs and leg restraints. I can't accept that it will ever be appropriate for the police to hood a disabled child, regardless of how they behave. I call on Sussex Police to stop doing this to children immediately."
The girl, who was held twice overnight in police cells, has "a neurological disability which can cause challenging behaviour", the IPCC said.
Her disability had not been diagnosed at the time of the police contact, but her mother told officers she believed she had an autism spectrum disorder.
The watchdog found that six custody sergeants had a case to answer for misconduct for failing to make sure that an appropriate adult was present; another for failing to make sure Child H was dealt with quickly while in custody, and two constables for using handcuffs. The officers were given "management advice" by Sussex Police.
Two more officers - a custody sergeant and an inspector - would have also had a case to answer for failing to make sure an appropriate adult was present but have since retired.
IPCC Commissioner Jennifer Izekor said: "This was a complex investigation, which found Sussex Police officers failed to respond effectively to the needs of a vulnerable child.
"While it is clear Child H had significant behavioural problems arising from her disability, Sussex Police and indeed other agencies which were - or should have been - involved, did not appear to have the skills and capacity to respond to her effectively."
Ms H's solicitor Gus Silverman from Irwin Mitchell said: "The systemic failings uncovered by the IPCC's investigation are truly shocking. Child H was detained for a cumulative total of 60 hours in various police stations. During this time every Sussex Police custody sergeant and inspector involved in her care failed to call an appropriate adult, most obviously her mother, to support her.
"In the last Queen's Speech the Government undertook to ban the use of police cells as places of safety for those under 18 years of age. This commitment must now be put in effect as soon as possible."
Temporary deputy chief constable of Sussex Police Robin Smith said: "We take our responsibility for any use of force very seriously particularly when it involves young people or those who are vulnerable.
"We welcome the IPCC's scrutiny and during its investigation the force has adopted many schemes to support vulnerable people and those with mental illness, learning disabilities and substance misuse issues.
"Aspects of our approach are being held as good practice nationally and we will respond to any new learning identified in the IPCC's report.
"As a direct result of the investigation into this case, personal safety and first aid training, which all officers have to undertake, has been updated.
"This means officers have learned communication skills to help them be more effective when helping people with mental illness. In addition all officers have refreshed their knowledge in the use of spit guards.
"As a chief officer I have a duty to protect officers and the public when we are called on for help, whether the threat comes from a child or someone who is unwell.
"This is very often the case and it was on several occasions that the girl's mother called for our help.
"The application of any type of restraint is considered only when the level of resistance causes concern for the safety of the detained person, the officer and other members of the public."