It appeared the children's throats were slashed while they were together in an upstairs bedroom, authorities said.
Two other girls, 14 and 9, were in the home at the time but were not injured, according to a statement from Camden County Prosecutor Warren Faulk and Camden Police Chief Scott Thomson.
The 14-year-old, Tiara, was watching Dominick, Amber, and their 9-year-old sister, Amy, because their mother, Debbie, was in Cooper University Hospital, recovering from surgery she had on Saturday, neighbors said.
Amber ran bleeding to a neighbor's house for help, neighbors said. She identified her attacker, but authorities declined to name the suspect, who had not been apprehended as of Sunday evening.
Dominick was pronounced dead in his home. Amber required surgery and was listed in critical condition at Cooper, authorities said.
Hours after the attacks, neighbors comforted one another and strolled along the block while firefighters washed blood off the front steps and sidewalk.
Pulliam, who has run the recreation program at the neighborhood community center for 20 years, couldn't get over that Dominick had just been dancing at the end-of-summer party on Thursday.
"He had fun all summer," she said. "He liked to play, he liked to have fun, he liked football."
Dominick, Amber, Amy, and a cousin spent their summer weekdays at the center, eating breakfast and lunch there and doing arts and crafts, Pulliam said. They went on field trips to the Mann Center for the Performing Arts, the Franklin Institute, the Adventure Aquarium, Clementon Park, and the Funplex.
The center, funded by the Camden Housing Authority, had filled backpacks with school supplies and haircut vouchers for the 35 or so children.
"I had a special black-and-gray one for him," Pulliam said of Dominick, "because he was small. He was only 6."
Authorities urged anyone with information to contact Camden County Prosecutor's Office Investigator Terry King at 856-225-8400 or Camden Police Detective Robert Chew at 856-757-7420.
Information can also be texted to firstname.lastname@example.org.
Cynthia Pulliam, director of a summer recreation program in Camden, talks about Dominick, the 6-year-old boy who was stabbed to death, at www.philly.com/camdenstabbing
Contact Bill Reed at email@example.com or 215-801-2964.