Who is the kid on Preview app icon?
Apple often uses employee's photos for things like this.

When filling iPhoto with content for on stage demos and later demos in their stores Apple will often request photos from their employees, through an open submission contest.  Obviously not every photo gets used but more often than not the family photos you're seeing are shared from an employee somewhere within Apple.  I've tried to find some of the old articles that outlined this process but they've either been buried away or removed at the request of Apple.

Knowing that, my best guess is this was a son, nephew, grandson, etc of an Apple employee around the time that Apple was looking to change the Preview icon.

Hardly a definite answer (we still don't know who he is) but it might be the closest we get.