Timmy Kinner is accused of attacking a group of children and the adults who tried to protect them at the party at an apartment complex that is home to many refugee families.
Fathi Mahamoud, 11, and Esrom Habte, 12, were playing outside with their friends during the party when they noticed a man approach. They told KBOI he began chasing and stabbing the victims.
"We saw him saying bad words and killing and stabbing our friends," the boys said.
"We didn't want to get stabbed," Esrom said.
"And then we ran to the closet," Fathi said.
Word of the child's death came at Kinner's first court appearance, where a judge told him that he was charged with first-degree murder and other felonies in connection with the Saturday night attack.
Kinner is American, and the victims are members of refugee families from Syria, Iraq and Ethiopia. Boise Police Chief William Bones said the evidence does not suggest the attack was a hate crime.
The suspect had recently stayed at the apartment complex but was asked to leave Friday over bad behavior, Bones said.
The 30-year-old, who is being held without bond, said he wanted to represent himself in court. But the judge ordered that he be appointed a public defender anyway.
Kinner had recently been a guest at the complex but was asked to leave Friday over bad behavior. He returned the next evening and began attacking people, police said.
CNN contributed to this report.