That really sucks. I hope those non-TERF estranged parents are able to find or form a supportive community somewhere, and maybe repair relationships with their kids eventually too, if and when that would be good for all involved.
What it says most to me is that those sites (at least the ones populated by bigoted/rascist/whatever parents) aren’t at all about repairing relationships so much as reasserting control. I also hope that the well-meaning parents out there figure it out, as much as they can anyway.