In an interview to air Friday on ABC's "20/20,"Elizabeth Griffin-Hall said that Powell slammed the door on her after the children were inside the house on Sunday. Hall said she banged on the door and heard Powell tell his son Charlie: "I've got a big surprise for you."
She also heard Braden, Charlie's younger brother, crying.
Authorities said Powell used a hatchet on his children then set a house fire that killed them all.
Powell's wife, Susan, vanished in Utah two years ago. Powell had long been a person of interest in the case but maintained that he had taken his sons - then 2 and 4 - on a midnight camping trip in freezing temperatures when she disappeared from their home.
Late Thursday, authorities told The Associated Press a computer in Powell's Utah home two years ago had computer-animated images that depicted "incestuous" sex. Pierce County Sheriff's Detective Ed Troyer said the images collected by investigators were realistic computer-generated depictions of incestuous parent-child relations.
The images were disconcerting enough that they prompted a psychologist to recommend last week that Powell undergo an intensive psychosexual evaluation.
An attorney for Powell's in-laws wasn't invited to see the materials before a custody hearing last week. Lawyer Steve Downing said he might have asked the court to change the terms of Powell's supervised visitation with the boys - if he had seen the images.
On Sunday, the social worker drove the boys from their maternal grandparents' home to their father's house outside Puyallup, about 35 miles from Seattle. Josh Powell lost custody of the boys last fall, after his father, with whom they lived, was arrested in a child pornography and voyeurism investigation.
Griffin-Hall said Charlie and Braden loved being with their father.
"One of them said what he wanted to do was go home and live with his daddy," she told ABC, adding that the boys would "light up" during visits with Josh Powell.
After Powell locked her out of the house, Griffin-Hall called 911 and her supervisor. Logs show deputies weren't dispatched until eight minutes after Griffin-Hall's initial contact with authorities, though police say any delay would not have stopped what ultimately happened to the boys.
The 911 dispatcher's handling of that call has been criticized, and an investigation has been launched into the emergency response.
The social worker said she told her boss "something terrible is happening here, and I was on the phone with ... when the house exploded.
"I wanted to get to the kids," she said. "I wanted to get to the kids. I would have broken in if I could."
But Griffin-Hall told ABC she doesn't think she could have saved them.
"How this happened is that Josh Powell was really, really evil. I couldn't have stopped him," she said. "I did everything I was supposed to do. I did everything right and the boys are still dead."
She said she loved the boys and was like a "grandma" to them.
"They are not going to grow up," Griffin-Hall said. "The world lost two beautiful boys to a monster."
