Cell phone video obtained by the ACLU captured one incident in which Sumner talks to a boy handcuffed in a chair. The boy is so small that he's handcuffed not around the wrists, but around his biceps.
"You don't get to swing at me like that," the deputy says as the boy cries. "You can do what we've asked you to, or you can suffer the consequences."
"Ow, that hurts!" the boy cries.
The deputy tells the boy what he'll need to do to get uncuffed.
"If you want the handcuffs off, you're going to have to behave and ask me nicely," he said. "And if you're behaving, I'll take them off, but as long as you're acting up, you're not going to get them off."
The handcuffs were removed after about 15 minutes, the ACLU said, citing school records.
The ACLU said the girl was twice handcuffed behind her back by her biceps and was also in pain.
"Both children were being punished for behavior related to their disabilities," the ACLU said in a statement.
All three incidents happened last fall.
Lawsuit calls for training, seeks damages
The group is suing on behalf of the two children, claiming Sumner violated the Americans with Disabilities Act.
The suit seeks a change in policies by the Kenton County Sheriff's Office and additional training for school resource officers in dealing with young children and children with special needs. It also seeks an unspecified amount of monetary damages against Sumner.
"Shackling children is not OK. It is traumatizing, and in this case it is also illegal," said Susan Mizner, disability counsel for the ACLU. "Using law enforcement to discipline students with disabilities only serves to traumatize children. It makes behavioral issues worse and interferes with the school's role in developing appropriate educational and behavioral plans for them."
CNN has reached out to Sumner's attorney for comment.
Sumner's attorney, Robert Sanders, told the Lexington Herald-Leader that Sumner put the children in handcuffs because "they were placing themselves and other people in danger of harm, and that's what the book says to do."
"Kevin Sumner is one of the best and most highly trained school resource officers in Kentucky," Sanders told the newspaper. "He's a teacher who left that profession to become a police officer. He's totally devoted to kids and schools and education."
Last year, the River City News featured Sumner in a profile lauding his educational experience. He was a teacher for four years before becoming a police officer, and he then joined those two skill sets as a resource officer, the newspaper wrote.
"It's good to have a positive interaction with the kids and to be a good role model and to undo any negative stereotypes of law enforcement that some of the kids may have," Sumner told the River City News.
Law specifies no physical restraint
According to an investigative report cited in the lawsuit, Sumner said the boy "swung his arm and attempted to strike him with his elbow," but the deputy blocked the boy's elbow with his hand.
Kenton County Sheriff Chuck Korzenborn is also named in the suit, accused of failing to properly train and supervise Sumner.
"Kentucky's school personnel are prohibited from using restraints, especially mechanical restraints, to punish children or as a way to force behavior compliance," Kim Tandy, executive director of the Children's Law Center, said in a statement. "These regulations include school resource officers. These are not situations where law enforcement action was necessary."
The Kenton County Sheriff's Office issued a statement in support of Sumner, stressing that "all the facts and circumstances have not yet been presented."
"School superintendents and administrators want, and need, to provide a safe environment for students and teachers. School personnel are permitted, like any other citizen, to request the assistance of a law enforcement deputy.
"Covington Schools' personnel requested assistance from the police during school hours after school administrators' efforts to de-escalate and defuse a threat to others had proven unsuccessful. Deputy Sumner responded to the call and did what he is sworn to do and in conformity with all constitutional and law enforcement standards," the office said.
An attorney for the families of the children handcuffed said the issue is now up to the courts.
"They've now agreed and admitted that this is a policy and that's why we have courts. We'll see what a jury in that jurisdiction believes and we'll see what the federal judge believes," Kenyon Meyer told CNN's "Erin Burnett OutFront."
State law prohibits the physical restraint of students and requires officers to do yearly training.
In a statement, Covington Independent Public Schools said it will not comment on the lawsuit, per district policy.
"However, the school district has fully cooperated with the children's legal counsel, as well as the Sheriff's Office in looking into the complaints and we will continue to do so," the statement said.
The school district added that school resource officers "are assigned in the schools to maintain the safety of students and staff" and "act in accordance with their training as law enforcement officers." But they "are not called upon by school district staff to punish or discipline a student who engages in a school-related offense."
The boy's mother said her son was traumatized from being handcuffed.
"It is heartbreaking to watch my little boy suffer because of this experience," she said, according to the ACLU.
"It's hard for him to sleep. He has anxiety, and he is scared of seeing the officer in the school. School should be a safe place for children. It should be a place they look forward to going to. Instead, this has turned into a continuing nightmare for my son."