Here is the Wikipedia's premise:
In the Tall Grass takes place when "after hearing a young boy’s cry for help, a sister and brother venture into a vast field of grass in Kansas but soon discover that there may be no way out."
BTW, You can read the excerpt of In The Tall Grass by clicking the title. Another awesome novella that would make a great feature film is The Box In the Spare Room and you can read that excerpt by clicking the title.