The children .. (spoiler)

Everybody here is talking about how bad the father was that he left the children with Grace, but:
1) this cult experience happened a long time ago, when Grace was really young and she was under medication ever since.

2)nobody, including viewers, knew that she was actually the person who killed all members of the cult. It is revealed only at the very end of the movie.

3. As for children, they were real cold-blooded sociopats. Of course, when I see it for the 1st time i felt great sympathy for them, even for what I assumed to be a bad joke.
But for the second time, you see all those red flags Aiden probably drugged Grace. He suddenly pretended to be nice by giving her a cup of tea). At the same time Mia pretended to have shivering in order to make his brother bring the gas heater - the potential cause of their death. The whole idea was not to show their father that Grace was a lunetic, like some posters suggests, but to drive her to suicide (like their mother did) and to usurp their father just for themselves. So they were driven by revenge, selfishness and jelausy. Real sinners.

So that is why the film is really good. It manipulates you into sympathizing with this "poor" catholic children, instead of feeling sorry for the mentally insane girl.
At the end, children got what they deserved.