I made a new document, 12 x 12. Then I brought in the cityscape paper, reduced it to the size you see. I gave the paper a deep inner shadow.
Next I use the pen tool to draw some trapezoid shapes for the top, bottom, sides to turn into masks for the various papers.
Once I had papers clipped to the shapes, I made a new layer above each shape mask/paper and used the brush tool to paint on some extra shadows (using a soft round brush, color 2c1901) along the seam edges (the edges where the trapezoids meet) and edges near the cityscape paper. I adjusted the opacity of the painted shadows until I liked how they looked. This was the step that really made it look more like a box. I added layers and more brushed shadows for additional depth.
I used shadows on their own layers and manipulated them for the boy and girl shadows. I did all this with PSE 11 (I'm sure it's much easier with PS).