The main thing you can improve on is your vocal range. At some point, all of the characters sound similar, even though I think there are supposed to be five characters. The mother's voice is too forced, and because of that she comes off very monotonous. I understand she's a vampire, but there's still a way to pull off a vampiric voice without sounding flat. She also sounds too much like a teenager.
The younger characters display better acting, but like I said, I was confused as to how many characters there are. I think you also did the voice of the girl that they sacrificed, but I confused her for the older child and the two younger. One of the child's voices (I think it's a boy?) changes by the end; you didn't maintain the raspy anime-boy voice long enough.
Now this part is delving into my own personal tastes, but I was put off while listening to most of the piece. I think it has to do with the writing. The youngest characters are perpetually frightened, the other child always sounds like a bored teenager, the mother is flat, of course the sacrifice chick is going to sound terrified. The thing is, they don't go through any particular emotional change. They display the same emotion throughout. Also, all the whining and screaming is grating on the ears after so long.
As for the mixing, there's not much differentiation in scene change, and I think the music is to blame. The same piece plays throughout, without fading in or out for scene change. There's some footsteps and the door opening, but there's no indication of time passing, so I was confused when the mother said, "Wake up, children."
Anyway, good luck in the contest :)