My grandfather's family were of South African nationality ( English mom, German dad by country of birth) and were visiting German relatives when the war began. My Grandfathers dad wouldn't join the party and so my granddad and his brother were put in an institution of some kind. He called it a boarding school for - as he put it " Bad Germans." To re-educate the children. It was by his account a horribly abusive place. He stayed there for a year or so until his dad rescued them. He never really recovered from it. Can anyone give me any information about these places?
I'm afraid there is no single term for this.
To re-educate the children.
This was basically any school in the Third Reich. All of them would (re)educate children with their propaganda.
It was by his account a horribly abusive place.
This also goes for all those places in that time. I don't think any kid thinks of the boarding school they were brought against their will as a good place. Coupled with the propaganda above and a focus on physical disciplinary action, it sure is hell. But there was no single special place or organization that one could name.
Even the Nazi's Elite went to boarding schools, Nationalpolitische Erziehungsanstalt called Napola, even those could be described as horribly abusive and this is where some of the Elite sent their children voluntarily.
There have been a few organizations to "deal" with kids in a more drastic way (Example, Example, no need to read if you want to sleep at night), but I don't think your relative was there. There was no coming back from those.
What you are looking for was probably called a Kinderheim, Erziehungsheim or Internat, all normal German words in use today. The Nazis simply took the existing organizations and infected them with their ideology.