Was the Dad the monster?

That's what I thought too at first, but since we barely saw the dad, it's hard to say. We don't know if he's really bad or not. They seemed to get along fine on the phone. It's a possibility, but in the end, I doubt it.

I thought there would have been a twist showing the daughter was really the monster.

I don't think you ever see the Dad in this movie. He seemed like a decent guy since the girl wanted to go live with him.

I thought the guy in the fort scene, for example, was just the mom's new boyfriend. As earlier in the movie they refer to a Roy, not a Dad, and the lil girl calls him an ass. So the only guy we ever see in flashbacks is mom's boyfriend, the fellow drunk, Roy. Not her Dad. From what I got.

I think it could be a metaphor for how terrible the little girl's living situation was with her trashy mom but i don't think there's enough there to assume that so it's more likely just a straight up monster story.