This is basically a vampire film, wrapped up in a family drama from Brad Anderson, previously responsible for Transsiberian and The Machinist. It struggles with identity along the way, confusing itself and the audience to some degree, but turns out to be certainly very interesting.
The family is made up of two households - mum (Jess) and dad (Patrick) splitting up because mum had previously been a junkie (for reasons the film never really goes into) and dad having started a new life with the family Nanny (Shelly). There's a girl (Tyler) and boy (Owen) being passed back and forward within a framework of legal access times and days.
The details of that last paragraph could well have made a stand alone film, family drama, in-fighting, difficult lawyer/client meetings, courtroom decisions about custody and so forth. There really was enough to work it like that - without making it into something of a Let The Right One In clone! Which is pretty much where it ends up.
The active part of the story involving vampirism is that the family's lovely cuddly dog heads off into the woods surrounding the new house that mum and kids have moved into, has an apparently strange experience out there (possibly involving a knackered old tree in the middle of a dried-up lake) and comes back a day or two later looking like a Cujo style monster-dog! He bites Owen. The dog is battered to death by mum - and Owen, whisked off to hospital.
Turns out, you guessed it, that Owen is now only interested in consuming blood, warm blood, human blood - preferably from living people (as it's warmer, presumably). However, much like in Let The Right One In, mum (and eventually daughter) work out what's going on and help Owen (blood is thicker than water!) to ensure that he ongoingly gets what he craves and needs to stay alive. Mind you, unlike Let The Right One In, he doesn't seem to have much of a problem with being out in sunlight! And that's about the bones of it really as we follow the family drama, interspersed with scenes relating to the goal of the nuclear family and some of the grizzly stuff that goes on in order to maintain the supply.
Michelle Monaghan (Black Site, Mission: Impossible) plays Jess very convincingly and is clearly the talent on show. The two kids aren't far behind, Finlay Wotjak-Hissong (The Banana Splits Movie, Land) and Skylar Morgan Jones. Skeet Ulrich (Scream) as angry dad doesn't have much to do and less so again, Nanny Danika Frederick. It's clearly a half-decent production though, so no low-budget cheapo horror/thriller like the recent Humpty Dumpty and Winnie the Pooh: Blood and Honey horror outings! It has some bite(!) and makes for good viewing. Slows down here and there when dealing with the family drama bits, but soon picks up again and back into the thick of it.
It's got lots of inconsistencies in the story - for one thing, we never really find out about the tree and what the mystery is surrounding the origins of all this - and Jess, who is a nurse, gets involved in some very dubious practices at work in order to feed-the-need (perhaps displaying some of the previously not covered addictive/obsessive tendencies) but I won't spoil that eye-opening stuff here! But OK - it's a story! It's certainly not fun - I don't remember a single laugh from any of the characters in the whole runtime, getting on for two hours - more like dark and menacing, by intent at least.
The direction was good, cinematography engaging with lots of scenery shots in and above the woods, inside and out of buildings - good use of light and shadow to add creep and suspense here and there. It's certainly worth watching, especially for the three main performances. And don't switch off too quickly at the end as there's a post-credits final scene!