Purcell, 54, yesterday appeared in Napier District Court and admitted the offending.
He pleaded guilty to two counts of assault on a child and one of common assault. The charges relate to multiple instances of abuse against the three boys.
His case was set to go to trial yesterday morning before guilty pleas were entered.
The 14-year-old boy was placed in his care in November 2010 before he ran away the following month.
He was returned to Purcell's home six days later, before fleeing a second time four days later.
At the time Purcell was a CYFs contracted caregiver employed by the Heretaunga Maori Executive in Hastings and cared for young people before the courts and from difficult backgrounds.
The violence included grabbing one boy by the throat, throwing him onto a couch, kicking, throwing a basketball, pulling hair out, punches to the arm and kicks to the back and stomach.
In addition, the children suffered verbal abuse and at various times were told to "shut up", "harden up", to stop being a "little bitch", "fat boy", "fat ****", "charity kid" and "little ******* peasant".
This occurred in late 2010 and early 2011.
Despite the now 16-year-old boy running away, notifying his father and authorities about the abuse and his obvious distress, he was returned to the home, his father told Hawke's Bay Today.
"How can they [the children] feel assured now? How can they be assured when you send them back there?"
He said words "could not describe" what he felt when told he was not capable of looking after his child, only for the boy to be assaulted under the care of a CYFs contracted caregiver.
The agencies responsible for sending the child back to Purcell's care should be held accountable, he said.
"I believe that they all played their own parts, they all should be held accountable, not just one guy. It wouldn't have happened again if they'd stopped it the first time."
If correct policies had been followed, and authorities had listened to the children's initial complaints, it would "never have happened", he said.
"That abuse could have been prevented."
Heretaunga Maori Executive Iwi social services co-ordinator Gordon Paku said yesterday he did not believe Purcell had committed the abuse and had pleaded guilty only to avoid prison.
"If one of my caregivers were guilty I'd sack him and report it to the police. I never, ever, believed Peter Purcell was guilty of anything."
Purcell was remanded on bail for sentencing in August.
CYFs refused to comment on the case.