In a film like this, it's somewhat strange to have one, but still: During one of the scenes where Kristi is clearly having a late-night chat with the demon, one of her 'responses' is "No, he's really sweet.". Think about it: Kristi was defending her father from a demonic entity. The demon, amazingly, is part of this. Dennis is only attacked at the end of the film, probably at the order/behest of the Coven. Other than that, the demon kept its word to Kristi.
A MUCH less heartwarming interpretation is the demon is simply going along as long as it's convenient.
That's probably it. Dennis wasn't much of a problem until the end - in fact, given the demon's MO, it's arguable that it saw him putting up the cameras and just thought 'hey, those could be useful in scaring the piss out of everybody! Thanks, man!' and let him be.
Happens a couple of times with Kristi. After stepping up and refusing to do what Toby asks her to (presumably the marriage ceremony), explaining she no longer wants to talk to him, she changes her mind and agrees when Toby locks Katie in the cupboard. Kristi agrees to go ahead with it, if he'll let Katie go. For a young child, that's a pretty brave move.
The Fourth Movie
There's a subtle one early on in the movie, at Wyatt's soccer game. Their dad shows up late, and he and their mom almost immediately start to argue. Alex gently leads Wyatt away, and starts teaching him how to do a cartwheel. It's very clear she's trying to keep him from watching their parents fight, and that this has probably happened plenty of times in the past.