EllenVanin
It won't realistically be possible possible identify and prosecute those responsible. Let me explain.
Indigenous residential schools were set up by various organizations at the behest of the Canadian government. The government's stated purpose was to "kill the Indian in the child". Eventually it became a requirement for Indigenous parents to send their children to residential schools, with authorities literally coming to reservations and rounding up children to be taken to school. While some children did have positive experiences, many suffered horrific abuses. Residential schools were in place for many, many years, with the last one closing in the early 1990s. Quite possibly, the children in this case died so long ago that those responsible are no longer alive.
Sadly, the impact of historical trauma on Indigenous people has been significant. Many survivors of residential schools went on to develop substance use disorders and other problems as a result of trauma. Often this led to their children being taken from them and raised by non-Indigenous foster families (e.g. the Sixties Scoop). This created further issues.
Thankfully, today the Canadian government has been taking steps towards redressing the harms done to Indigenous peoples. An example is the Truth and Reconciliation Commission. There is also a call to address cases of missing and murdered Indigenous women and girls (MMIW). Statistically Indigenous women are more likely to disappear or to be murdered than other women, often due to higher risk lifestyles resulting from historical trauma.
I'm Canadian and have some familiarity with the effects of colonialism on Indigenous peoples.