2d Gui for Kinect Restrict in Z axis

I'm trying to have some buttons for a kinect hand to hit. I've made a sphere a child of the hand, made it RigidBody, turn on is Kinematic and it hits my buttons fine. However I'd like the sphere to remain at a z pos of 0 so it becomes more like a curser and hits wont depend on the z position of the hand I've tried this

 using UnityEngine;
 using System.Collections;
 
 public class NewBehaviourScript : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
     
 
     }
     
     // Update is called once per frame
     void Update () {
 
     Vector3 temp = transform.position;
     temp.z = 0;
     transform.position = temp;
     }
 }

But the sphere is not restricted in any axis, I've also tried restricting on the rigid body, but presumably because the body has a parent that doesnt work either. Any ideas of how I can achieve this?