Depending on how much space you have, perhaps you could make the edge of a dark, twisted forest, looking like it's creeping in at one end of the plot? It ought to give a more darker feel, since you could imagine in a child's mind, bad thoughts creep in from the dark corners. You could use inspiration from red ridinghood, and build a tiny lit path going deeper into the forest, as the trees get more dense, and light levels decrease.
Fear of the unknown would be a cool idea to play with :)
Awesome plot so far!
