Most victims had been shot, police said, and at least one child had been stabbed.
"We just don't know the motive or cause of death," Memphis police Lt. Jerry Guin told The Commercial Appeal newspaper.
Children ages 7 and 4 and a 10-month-old baby were taken to Le Bonheur Children's Medical Center, Memphis Fire Department spokeswoman Melanie Young told The Associated Press. She did have details about the other victims, but police told the newspaper all the children were under the age of 12.
The Commercial Appeal reported that one of the children had been updated to stable condition and another upgraded to critical while the third remained in extremely critical condition. Hospital spokeswoman Jennilyn Utkov told The Associated Press the family has requested that no information be released on the children's conditions.
Authorities were called to the house by a relative who knocked on the door around 6 p.m. Monday but did not get an answer.
Wayne Bolden, who lives across the street from the scene of the shooting, said that a man who lived in the house periodically fired shots in the yard.
"He'd shoot on the 4th of July and New Year's Eve," Bolden said. "He'd have company over and I'd hear the shots."
He said the family there kept to themselves, but he would see four or five children from the house riding bicycles occasionally.
"It's a pretty quiet neighborhood; nothing of this magnitude has ever happened here," Bolden said.
Rob Robinson, who owns the brick, single-family home where the shootings occurred, told The Commercial Appeal the house was rented, and that a man, his girlfriend, a baby, and at least one other child lived there.
"They were very nice, very polite to me," Robinson said, adding that the man paid $550 in cash every month for rent. "It's kind of surprising, actually. I've never had any trouble with them, no damage to the property. They paid their rent and even helped with repairs and stuff."
Police were uncertain whether the man was among the victims. Robinson said the man living at the home had problems with a former girlfriend. "When I was over there, there were a lot of heated conversations over the phone," he said.
Neighbors said there had been a disturbance over the weekend, and a woman was outisde the home, blowing her car horn.