A pleasant, short, easy read, enlivened by occasional illustrations.
It was perhaps more frightening for me than the average child, I suspect, since at one point early on Araminta and Wanda were left in a position where they could easily have died (at the bottom of a pit in a cave on a deserted island, and no one except the two horrible children who tricked them and then pulled out the ladder know they're there). I pictured them lingering, desperate for help, only to succumb to dehydration a few days later. I didn't think it was likely, given "children's book," but I didn't like how casually they treated what seemed to me to be a life-and-death situation.
That aside, my other demerit point would be for its essential niceness. It's almost too nice, or too conventionally nice, given the ghost, vampires, etc., and I'd like for a bit more of the Addams Family or Edward Gorey (or even Lois Lowry) vibe here. It's like if the Bobbsey Twins wore more black, but that's about it. Where's the Spookiness?
(Note: I'm a writer, so I suffer when I offer fewer than five stars. But these aren't ratings of quality, they're a subjective account of how much I liked the book: 5* = an unalloyed pleasure from start to finish, 4* = really enjoyed it, 3* = readable but not thrilling, 2* = disappointing, and 1* = hated it.)