Well, I am not a portrait person but I took a photo of my grandson last week in almost identical lighting so I do have recent experience wrangling with this dilemma. I think it is adorable photo. It's sharply captured, with no distractions thanks to blurred background, the model is appealing and there is a bit of a theme (thanks to the hat). For my taste, the photo is too dark. If I wanted to keep it in sepia (which surely fits that hat!) I would brighten it first, then deal with the flatness by upping contrast, clarity and blacks. Then I'd wrestle with the over-light ear, the whites of the eyes and the teeth with careful/slight local adjustments. I might experiment with other tones of sepia to see what works best. There's a few little "things" that may have been caused by the lighting and may be just as well to leave alone but might merit experiments as they show more once the photo is brightened - what looks like a black eye beneath one eye, and an odd shadow under the other, the triangle of bright white on the face & ear, a dark spot to the left of the mouth and another beneath that spot. Of course any change would work better from your original file. I think you've got a photo worth entering, but I don't think the overly dark treatment does it justice.