The American Civil Liberties Union (ACLU) is enraged after a video came out showing a deputy from a sheriff's office in Kentucky handcuffing two crying children with disabilities inside a school in an incident that happened last year.

ACLU, together with the Children's Law Center and Dinsmore & Shohl, filed a federal lawsuit on behalf of the two aggrieved children against the Kenton County Sheriff's Office in Covington, Kentucky.

The complaint alleges that a deputy from the sheriff's office unlawfully handcuffed an eight-year-old boy and a nine-year-old girl, both of whom have disabilities, in the fall of 2014. The ACLU believes that the deputy sheriff involved in the incident violated the US Constitution and Americans with Disabilities Act.

"As a result of being subjected to unnecessary and excessive handcuffing, plaintiffs experienced pain, fear, and emotional trauma, and an exacerbation of their disabilities," the lawsuit reads.

Col. Pat Morgan, a spokesman for the Kenton County Sheriff's Office, told The Huffington Post that they are still waiting for their own lawyers to review the lawsuit prior to making a statement regarding the matter.

The ACLU released the video of the handcuffed boy, who reportedly has attention deficit hyperactivity disorder and a history of trauma. The video shows the boy crying inside a classroom while the deputy sheriff handcuffs his biceps behind his back. The boy cries, "Ow that hurts!" but the law enforcer ignores this and continues handcuffing the boy who is told to on a chair with the handcuffs on for 15 minutes.

The officer then tells the boy, who stands 3 1/2 feet tall and weighs 52 pounds, "You don't get to swing at me like that. Now sit down in the chair like I've asked you to."

At least 100,000 children are shackled in the US every year, said David Shapiro, a campaign manager at the Campaign Against Indiscriminate Juvenile Shackling. He said these shackles are used in schools, treatment centres, and courtrooms, according to Mother Jones.

In 2009, the US Government Accountability Office (GAO) reported that it found hundreds of cases of alleged abuse and death involving the use of shackles and seclusion on school children over a two-decade period. At least 20 of these cases resulted in death.

Officials said one of the cases was that of a seven-year-old girl who died at a private treatment centre after being restrained face-down on the floor for hours. "The staff was allegedly unaware she had stopped breathing until they rolled her limp body over and discovered she had begun to turn blue," the GAO said.