When I take a new student, especially an adult, but also a young student who has been learning the violin in school, I start them out with Six Steps to Perfect Posture. This exercise is guaranteed to make a new student look at me askance, but if he's willing to practice this, I know that he'll succeed in my studio. A very young student probably won't need this, especially if it's the very first lesson. In that case, a simple "zip, step, sniff, bouncy knees" will suffice. Kids are so loose and flexible before they get older and filled with angst. :-)

The purpose of the Six Steps is to use your breath to put your body in a comfortable position. Some steps will feel very uncomfortable when you are just starting.