To prosecutors, the girl is the sole eyewitness in their case against the mother's live-in boyfriend, James Embre, 26, who is on trial on a murder charge. They say Embre, angry about having to walk the child home from daycare on April 17, 2018, was later smoking marijuana and playing video games with the children's mother when he went to another room and beat the boy.
But Embre's attorney, Jonathan Zucker, put forward a shocking defence. He told jurors that his client did not hit Ace and argued that the boy was killed by someone else in the apartment in Southeast Washington - either his sister or his mother.
"This is a very disturbed child who had a habit of hurting Ace," Zucker said of the 7-year-old girl. Her accusations, which he called "fantasy and fabrication," are the only evidence against his client, he added.
It is rare for a child so young to be a witness in a murder trial, but with her account critical to both sides, the second-grader has been subject to intense questioning. Already on the stand for two days, she is set to return Monday.
Both Zucker and Cynthia Wright, an assistant US attorney who specialises in cases involving the deaths of infants and young children, have had a hard time helping the girl focus during hours of testimony. At one point, Judge Juliet McKenna encouraged the girl to take a break and run around the courthouse halls with Wright.
What quickly became apparent was that the girl's testimony was often contradictory. At one point, she said Embre never hit her or her brother. Then she said Embre hit her brother on his back. Then she said Embre hit her brother on his stomach. Then she repeated that Embre never hit them.
Zucker played videos in which the girl was recorded saying that Embre struck her and her brother. But on the stand, depending on how Zucker or Wright asked questions, she either elaborated on her assertions or contradicted them, saying at points that she did not know why she made her earlier statements or that she was being "dramatic."
The girl often shrugged her shoulders, prompting McKenna to remind her to "use your words."
Before the trial started, Zucker objected to Wright calling the 7-year-old as a witness because of the girl's age. Wright argued that Zucker should not be allowed to name Ace's sister or the children's mother as alternate suspects, saying there was no reliable evidence against them. McKenna overruled them both.
Wright told jurors in her opening statement that Embre first became angry that he had to walk Ace about 40 minutes from his daycare that April evening to their apartment in Southeast Washington.
Later that night, the prosecutor said, Embre and the children's mother, Ziykillya Thurman-Ahmad, were in her bedroom smoking and drinking. Embre repeatedly went into the bedroom where Ace and his sister were, Wright said, and beat the boy.
At one point, Wright said, Embre rushed back and said Ace was not breathing and had vomited.
Wright told the jury that Ace had fractures to his ribs and that there was evidence of prior bruising. She said he bled internally from a lacerated liver and an artery that supplies blood to the heart.
"In less than two hours, this beautiful, healthy child was dead," Wright said, showing two poster-size photos. One showed Ace smiling; the other showed him in the morgue.
Zucker told jurors that Embre had no history of abusing the boy.
The attorney turned his focus to the others who had been in the apartment. The boy's sister had been disciplined in school for getting into fights when she was younger, Zucker said. He said her mother told authorities that the girl once gave her brother a black eye.
After Ace died, Zucker said, the girl was temporarily placed in a foster home as the case was investigated. He said the foster mother had the girl removed because she was hitting other children.
Zucker also told the jury that the children's mother should be considered a possible suspect. Thurman-Ahmad has a history of mental illness, illegal drug use and was a battered child herself, Zucker said. He said she had told authorities that to discipline the children, she had previously struck Ace with her hand and her daughter with an electric cord.
Authorities were alerted to Ace's injuries when Thurman-Ahmad, who also is expected to testify, called 911 and then ran outside and flagged down a police officer.
As dramatic body-camera video was shown to the jury, DC police officer Jeffrey Buchanan described how he and his colleagues tried to revive the child in the back of a squad car.
"Come on, big guy. Come on," one of the officers in the video implores. Ace, wearing a yellow T-shirt and training shorts, appeared lifeless as one of the officers stuck two fingers down his throat to see whether anything was blocking the toddler's airway. Then the officer started CPR, rhythmically pushing on the boy's chest, turning him over and then pushing on his back.
The officer who performed the CPR, is "pretty banged up. Real emotional," Buchanan testified, his voice cracking.
Seconds later on the video, sirens from the ambulance are heard in the background. Ace, accompanied by Embre, was taken to a hospital.
Thurman-Ahmad and her daughter followed in another police car.
Jurors, who were played the body-camera video from the driver of that car, heard Thurman-Ahmad yelling and questioning her daughter.
"What happened?" Thurman-Ahmad asks.
"I don't know," the girl responds.
"What do you mean you don't know? You were in there with Ace," Thurman-Ahmad says. "Did James do something? What did he do? Did he whup Ace? Tell me. What did he do?"
Her daughter responds, "I don't know."
Thurman-Ahmad yells back, "You can't say you don't know."
Her daughter then says Embre had his "mad face" on when he came into the room and that he "punched Ace." The girl said Embre punched her in her leg and told her not to say anything to her mother.
Earlier, while still in the apartment, the girl told a police officer and her mother that Ace twice fell out of the lower bunk bed onto the floor.
Embre's lawyer insists a single punch would not have resulted in multiple injuries to the boy. Zucker argued that the evidence shows that Ace was beaten numerous times previously, as well as on the night he died.
In court, Zucker tried to get the girl to admit to harming her brother, either on purpose or by accident.
"I never hurted Ace," she said with a raised voice.
"Did you ever jump or stand on Ace?" Zucker asked.
"How do I jump on a 2-year-old?" the girl asked. "I never standed on him either."
When the girl finished testifying late Thursday, she walked to Zucker, hugged him and gave him a picture she had coloured while she was testifying. Then she hugged Wright.
