The workflow, for any given scene on any given show, goes something like this: everybody gathers around while the "first team" (the actors) read through the scene and the director works out the "blocking" (i.e., their movement and the camera's movement) for the scene (which usually is broken down in to many shots).
The first team is then excused to the care of the makeup and costume departments, and the "second team," the stand-ins, are called to replace the actors. Stand-ins are hired to approximate the look of the actors (size, shape, hair color), so that the lighting and camera team can prepare the shot and be ready to go when the "first team" is called back to the set.
Children work in Hollywood under strict labor laws -- the hours of their work days are sharply limited, and they have to complete a certain number of hours of "school" with a company tutor, each day. Thus the company looks for strategies to work around them as much as possible -- one of those strategies is to hire "little people" -- adults who are small in stature -- to stand in for the kids.
Here Kevin Thompson stands in for Erik Per Andersen (Dewey) on "Malcolm In the Middle." Erik was seven when the show commenced and when Kevin was hired; this shot was taken in season seven, when Erik was 14, and had outgrown his double -- thus Kevin would stand on a box, as you see here, or wore special shoes with hugely elevated soles.
