Thanks so much for everyone's feedback. And Splinter, thanks so much for sharing: those car scenes are so precisely what happens to me every single time too! It sounds painfully familiar. I'm really torn. I love her and the thought of leaving her to this self-destruction pains me.
And even aside from that: my (it feels like they're mine) children were already abandoned once by their (worthless) biological father. When I first met them, they were shy, struggling in school, scared of further abandonment. Today they are happy, they've adjusted to a family life that is - while problematic - far better than what they came from. We read together, work on schoolwork together, eat family meals together. They graduated at the top of their class this year.
I didn't mean to deny in my earlier post that they don't suffer from what is happening - I am very aware that this leaves scars that are hard to erase. But they are not witnessing the worst of it, and I have done the best that I can do to make sure they have everything they need. And I don't know that I could forgive myself for abandoning them now. Moving them away from here will be hard, but it will also come with benefits: their current schools are mostly worthless, there is no support system here from which they can benefit, and this would be radically different afterwards: we would be in one of the best school districts in the country. Their grandparents, the family to which they are closest, will be precisely as far away from them then as they are now, only in the opposite direction. I feel like whatever harm may come from moving them is not outweighed by the harm of leaving them alone in this situation.
But maybe I'm just rationalizing.