I just wanted to say that I am skeptical of this portion of the narrative.

Were the graves really unmarked? Or were they marked with wood crosses or wood markings that may have been removed (by vandals) or simply rotted away over time as the graveyard became overgrown due to not being kept up? Have we forgotten that these schools date back to the 1800's? This was a time when not everyone had a car or even a telephone.

I've searched all the news stories and I have yet to see any photographs of the site itself. It's just being fed to us that they were unmarked and assumed that it was to cover up mistreatment.

So many assumptions are being made without even looking at the death records (which do exist, it would just be a lot of work to dig them out of non-digital archives).

As for why the kids were not returned to their parents for burial, the answer is that in cases where parents could not be located, the residential schools became the sole caregiver of the children meaning there would not have been a family to return the bodies too. So, naturally, they would have done their best to provide the dead some sort of burial.