The news reports of this seem to imply that the kids were killed. In all likelihood they mostly died from communicable diseases as was quite common in those days. Some of them may have died in actual pandemics, where the sheer quantity of dead called for mass graves.
This is a major scandal in Canada, aboriginal children were assimilated and those who resisted met an untimely end. In one case, the royal family visited a school and left taking a number of students with them, students who were never seen again according to witnesses at the time. And now this, 215 children's remains found on the school grounds. Canadian priest Kevin Annett has worked extensively trying to bring this information to the attention of the world.
https://topdocumentaryfilms.com/unrepentant-kevin-annett-canadas-genocide/
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