child not moving with parent

very basic here: i want to be able to instantiate cubes as childs of my gameObj and move them toghether. this is just testcode: i want to instantiate a cube object and then apply a force to my current object. the cube should follow, right? no, it doesnt. (gravity is off)

public Transform cube; Transform newCube; Rigidbody rb;

 void Start ()
 {
     rb = GetComponent<Rigidbody>();
     newCube = (Transform)Instantiate(cube, transform.position + Vector3.left, transform.rotation);
     newCube.parent = transform;
 }

 void FixedUpdate()
 {
     rb.AddForce(Vector3.forward);
 }