I wouldn't say there is an appropriate reaction, but some of the efforts to escape and fight just seemed out of place or lacking. Like they all acted like there was no where to run before they actually knew there wasn't so they didn't put in full effort. Like when the kid personality was in the room whispering in the girls ear, the other two didn't run outside the door. None of them thought to jump him while in that personality. No ambushes, no urgency, no experimentation. Or even no one noticing it wasn't the real father who got into the car. Or the doctor randomly showing up to his place, or the doctor not playing it smarter when she realized the girls were in the closet.
Everything about the movie was too egregious to suspend belief.