Cool, that gives us a good idea of what's going on!
What is the kid doing with his cards at the beginning? He takes a big swing at something? If he's picking up a card then I would change that gesture. Something that big and fast could make the cards fall and you'd think that the kid is very careful about his creation and doesn't want anything to break.
I would put the cards more screen left so that they are clearly visible.
With the cards on the left the kid should pick them up very delicately and since the cards are screen left the silhouette of the kid's arm/hand/fingers will be nice and clear.
What's tricky with a walk/run cycle is that the speed is constant when you have him move around. If you'd have him slow at the beginning and speed up towards the end the feet would slide. But that's what I would suggest for the background guy. If possible, try to add rhythm and contrast to his movements, so that he isn't just running by with even timing step wise. Hope that makes sense.