"We've seen some as young as 12," MacDonald said Wednesday after releasing her annual report.
"That has been very concerning for our office, too. We have never seen that age group before."
There were 17 suicides among young people in the 2012-13 fiscal year. Eleven involved kids in the province's child-welfare system. Four were just 12 years old, the report said.
Preventing suicides has to involve more than child welfare agencies, MacDonald added. Her report calls on the provincial and federal governments to come up with new efforts, partly to ensure services are available to a younger audience.
MacDonald's annual reports focus largely on the province's troubled child-welfare system, which has seen the number of kids in care skyrocket in the last decade to 9,730. The vast majority are aboriginal. The Office of the Children's Advocate is an independent agency that acts as a voice for children in the system.
MacDonald's report touches on themes that have been raised before, notably during the public inquiry into the 2005 death of Phoenix Sinclair. She was a five-year-old girl who was tortured and beaten to death by her mother and mother's boyfriend after social workers decided the little girl did not need help and closed her file.
One of MacDonald's predecessors told a legislature committee in 2010 that child welfare in the province was in a state of chaos.
Among MacDonald's top concerns is that some risk assessments are not being done properly, meaning authorities do not get an accurate picture of whether children are safe in their homes. An information guide for social workers doing assessments is cumbersome and some workers are not properly trained on how to use it, MacDonald said.
"It's not user-friendly, and so we'd like to see better technology being used."
The report also says some of the problems in child welfare are tied to inadequate housing, especially in remote communities.
"In our visits to First Nations communities, numerous housing problems have been identified by community members, including overcrowding due to insufficient houses to meet the needs of the community, lack of foster homes due to housing not meeting the standards required," the report states.
"In some communities, a lack of sewer and water service in many homes makes it difficult to maintain sanitary conditions necessary to the safety of very young or medically fragile children."
Manitoba Family Services Minister Kerri Irvin-Ross, who took over the portfolio last month, was not available for comment Wednesday.
Her predecessor, Jennifer Howard, has said the government has invested in more front-line social workers and acted upon the recommendations of reviews.