Transform.Find  

function Find (name : String) : Transform

Description

Finds a child by name and returns it.

If no child with name can be found, null is returned. If name contains a '/' character it will traverse the hierarchy like a path name.

// The magical rotating finger
function Update() {
    aFinger = transform.Find("LeftShoulder/Arm/Hand/Finger");
    aFinger.Rotate(Time.deltaTime*20, 0, 0);
}