An 11-year-old girl with a neurological disability was handcuffed and restrained with a mesh hood and leg straps while being held in custody by Sussex Police, the police watchdog found.
The child was arrested by Sussex Police for minor offences and held for 60 hours without an appropriate adult.
The watchdog said a number of officers and staff had a case to answer for misconduct.
The independent Police Complaints Commission made a number of recommendations after its investigation, including: Improved training on the use of force on children and adults with mental illness, to ensure the use of force is avoided wherever possible, additional training on detaining vulnerable people and the role of an appropriate adult and ensuring officers are accountable for their use of force.