I was looking into putting a prosthetic arm on the son to parallel the "prosthetic world" that his father placed him in. It could also be an artifact of an earlier event, like the accident that killed his mother who died saving his life (when he was an infant). I also like playing with the contrast and irony, of how this little boy has a prosthetic arm that seems more full of life than the virtual copies of people walking on the streets.
I decided to put a "backpack" or a bag on the father, instead of a "brief case," because I wanted him to seem more casual, less impersonal than the other adults walking in his son's world. I could imagine him pulling out a snack out of his bag for his son, or the toy that he keeps for him while out in public, etc.
To stand out from the rest of the world, none of the main characters have shades or face obstructing features.