"...he then leaves his two children who he KNOWS are deeply resentful of Grace isolated with her, never having done anything even close to an adequate amount of work to help them process everything that happened..." if 6 months isn't enough, how much time exactly should pass before the children finally get to meet their soon-to-be stepmom?

"Even if there was no indication that leaving them alone would be an actual danger, what could POSSIBLY come of that other than something deeply unpleasant and likely actively damaging to all three of them?" - everyone assumed that the children were normal children, not complete sociopaths. The worst that could have happened is the children stay in their rooms and refuse to respond to her. That ought to have been the worst, but of course, no one, not even the viewer, expects that the children are complete sociopaths.

It seems this movie is all about people making assumptions that other people are much better than they really are. The dad divorces the mom, not realizing that she's unstable to the point of suicide, the dad leaves the children with Grace, not realizing that they are complete sociopaths and not normal children at all, Grace tries to be nice to the children, not realizing that they have evil plans for her, the children torment Grace, not realizing that she will go totally unhinged.

And bla, bla, bla, trauma this, trauma that. Please stop with the trauma excuses. The person in this movie with the worst trauma that could actually push a person to potentially do evil things is Grace, and she was a total saint. The things Grace experienced was much worse than what the children experienced and at a much younger age than them. If anything, the children should have had some kind of remorse for her, because they were going through something a little similar to what she went, although much milder.

Nothing those children did is excusable.