How to move a child object by drag and drop? Please help me :(

Hey guys, so i have an object on a plane and i want to move it on the x and z axis by using drag and drop with my mouse. I converted x,y to x,z by rotating the vector3. But i want the object to be a child of an empty object, so i can use the empty one as a kind of folder to store all object inside. The problem is i can access the objects with the OnMouseDrag function, or i made a mistake. Can someone check my code and help me pls :) Thanks in advance

Regards,

Here is my code:

 public class DragAndDrop : Interactable {

 Vector3 lastPosition;
 Vector3 currentPosition;
 Interactable interactable;
 Ray ray;
 RaycastHit hit;

 void Update() {
     FindObject ();
     MoveSelectedObject ();
 }

 void FindObject() {
     if (Input.GetMouseButtonDown (0)) {
         ray = Camera.main.ScreenPointToRay (Input.mousePosition);
         if (Physics.Raycast (ray, out hit)) {
             interactable = hit.collider.GetComponent<Interactable> ();
             if (interactable != null) {
                 Debug.Log ("Hit! Interactable: " + hit.collider.name + hit.collider.transform.localPosition);
                 lastPosition = Camera.main.WorldToScreenPoint (hit.collider.transform.localPosition);
             }
         }
     }
 }

 void OnMouseDown() {            
 }

 void MoveSelectedObject() {
     if (interactable != null && Input.GetMouseButtonDown (0)) {
         currentPosition = Camera.main.ScreenToWorldPoint (new Vector3 (Input.mousePosition.x, Input.mousePosition.y, lastPosition.z));
         currentPosition = Quaternion.Euler (90, 0, 0) * currentPosition;
         currentPosition.y = 0;
         currentPosition.x = Mathf.Round (currentPosition.x);
         currentPosition.z = Mathf.Round (currentPosition.z);
         hit.collider.transform.localPosition = currentPosition;
     }
 }

}