The perspective makes the little girl appear almost like a giant (or the people like dolls), so you might want to lay out a basic perspective grid first. I really like the idea of the piece and how you have the dead chicken in the foreground which helps give it a sense of depth. The pose of the girl seems a little unnatural to me, perhaps having her head faced down towards the ground would show her sadness/shock more, or having one arm whip away the tears whilst the other arm is holding her up would be a bit more dynamic.
I wouldn't worry too much about it been too dark at this stage. Just focus on getting a good sketch first (of course keep in mind how you want the light to play out whilst you set up the scene). When you do focus on the lighting I think that having a silhouetted dead chicken works really well and maybe having part of the girl under a cast shadow and partially lit with cool light through the trees would look interesting too. Also, it might be cool to have some of the feathers leading your eye towards the dad and dog who are way off in the background.
Look forward to seeing how it goes, good luck