A few questions... *spoilers*
1) At the end of the film, when Thackery is a ghost, he seems to realize the curse has been lifted and his soul set free due the witches being dead.
Why, then, didn't he realize the sisters weren't dead after Max and the others torched them in kiln at school? He was still a cat. So he should've known that he hadn't been set free, and the witches were not dead. Unless, the only reason he was set free at the end was because his cat body happened to die right at the same time the witches did.
2) Billy seems to be legitimately trying to catch the kids through a good portion of the film. He is seen following them through the sewers, etc. Then, out of nowhere, toward the end of the movie he grabs Max only for his knife and cuts the threads holding his lips together so that he can call off to Winnie. So, was he ever actually after them? If not, why was he chasing them? He couldn't have known Max had a knife for his threads....
3) Does the number of children's life forces equate the length of the time that the sisters will get to live? In other words, would they have to continuously keep sucking the life force from children in order to stay alive?