Bob Pringle released a report Wednesday after an investigation into the death of a child who drowned in a bathtub in 2010.
He said the toddler, referred to as "Mark," was placed in an overcrowded home and became the fifth child under the age of four to live there.
Pringle found that the child's death was preventable and that Social Services failed to act as a "prudent parent."
"This boy was forgotten about," he said.
A coroner determined the 22-month-old accidentally drowned. The foster mother was charged with criminal negligence causing death but was found not guilty in court.
Pringle said the foster parents had received approval to care for two children in Alberta. In Saskatchewan, three children were placed with them immediately without a new home study. Before Mark died, Social Services had approved the family to take a sixth child, who was 18 months old.
"You can see a disaster looming. Just think about going to get groceries or trying to take a child to the doctor ... how do you even do that logistically?"
He said Mark died after his foster mother ran a bath and left him unattended when another child's demands called her away.
"Somehow ... he flipped the hot water tap on," he said. "The idea of him drowning and getting scalded is hard to even think about, but that's what happened."
Pringle's report points to a lack of quality case management and supervision as well as a failure to comply with policy in general.
He suggests Social Services write a letter of apology to Mark's parents that recognizes the shortfalls that occurred when he was placed in care. The letter should include that Social Services didn't immediately notify Mark's parents of his death or prepare them for how he died, Pringle said.
The children's advocate made nine recommendations, including better staff training and a review of the foster home program.
Social Services Minister Donna Harpauer apologized to Mark's family at a press conference Wednesday.
"We failed to act in Mark's best interests and we failed to act in the best interest of his family," she said.
She added that the agency takes responsibility for placing too many children in the home.
"This home was overcrowded and should not have been," she said.
She said there are now fewer children in care and front-line workers have been added since 2010.
"We also want to look at spreading the workload evenly," she said. "Our commitment is there to strengthen the system."
Pringle also mentioned relevant recommendations from other reports: looking at caseloads for social workers and introducing better accountability measures.
Last month, Pringle said inconsistency in casework is a problem in child protective services. He was responding to the outcome of a coroner's inquest into the death of six-year-old Lee Bonneau, who was in foster care when he was killed by an older child in 2013.
Pringle's own report on that case found the 10-year-old boy who killed Lee had behavioural issues and probably should not have been in the community unsupervised. He had a history of issues and Yorkton Tribal Council Child and Family Services had been involved, but the child was still living with his family.
He said the inquest made it clear that agencies need to communicate better with each other and social workers are struggling under heavy work loads.
"It's not about the foster family, it's about the system not supporting them."
— By Clare Clancy in Regina
Note to readers: This is a corrected story. A previous version said the ministry.