I'm curious of how the horror genre is holding up in the children's market. Is there room for another R.L. Stine, or is that era past? If not, who would be the best publisher to approach with a middle-grade horror story? Any ideas?Is that era past? Have you not noticed the hordes of zombies currently overrunning the shelves?
Horror, like any genre, is fairly perennial. Little Brown is doing very well with its Cirque du Freak series. Simon & Schuster just got a Printz Honor for Monstrumologist. Harper could certainly be doing more with the popular Scary Stories to Tell in the Dark series, but maybe they're satisfied with those sales. Etc.
I would suggest you do some more market research, and try one of the few publishers who don't currently have a strong horror title or series on their list. And then try the rest of them.