I think this piece is so lovely, and most of my thoughts have been covered already. Another thing you might try and see how you like them is to add a bit of bounce light on the tree and girl and dog to add even more contracts to them. I also thought maybe you could give us something of interest towards the bottom of the screen that adds to the story- right now everything interesting happens in the upper 1/3 of the image and there's nothing else to really lead us around the rest of that space, other than your lovely painting. (Very slap-dash) thoughts I had were low-contrast fireflies, a dark cat climbing the ladder, a sign tacked to the lower left tree that says "no boys allowed" or something like that, a few lizards or mice climbing up the ladder, more bats. Alternatively, if you want her to be the center of the image and for the scene to feel serene, solid and strong, you could bring the action down more toward the center of the page, or extend the painting up a bit further.