Adjusting Z value of Child Cam in Unity3Ds Follow Camera prefab? (FreeLookCam.cs)

I've been using Unity3ds standard Free Look Cam, and it's been working great so far for what I need, but I'm having a hard time implementing a zoom.

From what I'm understanding, the parent Camera's Y rotation moves...then the child from that is the Pivot object and that's X position moves...and then the child of that is the child Camera object, the camera that's used, and that one's Z determines the distance that the camera follows.

When the game starts, I'm not really wrapping my head around why you can't simply adjust the Z value of the child Camera that's used without it messing up and losing it's lock on it's object that it's supposed to focus on.

If I hit "play mode" and just manually enter in a different Z position value for the child camera, it works perfectly, and yet I can't put this code in the child cam's update method...nor am I finding where I should put it or how I should put it in the following code.

Ultimately, I'm trying to implement a camera movement based zoom, and trying to stay away from field of view zoom.

Here's just some test code that I was trying to put inside a script attached to the child camera:

 void Update() { 
     transform.position = new Vector3(0,0,-7);
 }

Without it somehow interfering with FreeLookCam.cs's HandleRotationMovement method (I think that's what's happening)...

 using System;
 using UnityEngine;
 using UnityStandardAssets.CrossPlatformInput;

 namespace UnityStandardAssets.Cameras
 {
 public class FreeLookCam : PivotBasedCameraRig
 {
     // This script is designed to be placed on the root object of a camera rig,
     // comprising 3 gameobjects, each parented to the next:

     //     Camera Rig
     //         Pivot
     //             Camera

     private int MainCameraChildID;

     [SerializeField] private float m_MoveSpeed = 1f;                      // How fast the rig will move to keep up with the target's position.
     [Range(0f, 10f)] [SerializeField] private float m_TurnSpeed = 1.5f;   // How fast the rig will rotate from user input.
     [SerializeField] private float m_TurnSmoothing = 0.0f;                // How much smoothing to apply to the turn input, to reduce mouse-turn jerkiness
     [SerializeField] private float m_TiltMax = 75f;                       // The maximum value of the x axis rotation of the pivot.
     [SerializeField] private float m_TiltMin = 45f;                       // The minimum value of the x axis rotation of the pivot.
     [SerializeField] private bool m_LockCursor = false;                   // Whether the cursor should be hidden and locked.
     [SerializeField] private bool m_VerticalAutoReturn = false;           // set wether or not the vertical axis should auto return

     private float m_LookAngle;                    // The rig's y axis rotation.
     private float m_TiltAngle;                    // The pivot's x axis rotation.
     private const float k_LookDistance = 100f;    // How far in front of the pivot the character's look target is.
     private Vector3 m_PivotEulers;
     private Quaternion m_PivotTargetRot;
     private Quaternion m_TransformTargetRot;

     private float CamXp = 0;
     private float CamYp = 0;
     private float CamZp = -2;
     public float cameraDistanceMax = 5f;
     public float cameraDistanceMin = -1f;
     public float cameraDistance = 2f;
     public float scrollSpeed = 2.4f;

     protected override void Awake()
     {
         base.Awake();
         // Lock or unlock the cursor.
         Cursor.lockState = m_LockCursor ? CursorLockMode.Locked : CursorLockMode.None;
         Cursor.visible = !m_LockCursor;
         m_PivotEulers = m_Pivot.rotation.eulerAngles;

         m_PivotTargetRot = m_Pivot.transform.localRotation;
         m_TransformTargetRot = transform.localRotation;
     }


     protected void Update()
     {
 
         HandleRotationMovement();
         if (m_LockCursor && Input.GetMouseButtonUp(0))
         {
             Cursor.lockState = m_LockCursor ? CursorLockMode.Locked : CursorLockMode.None;
             Cursor.visible = !m_LockCursor;
         }

     }

     private void OnDisable()
     {
         Cursor.lockState = CursorLockMode.None;
         Cursor.visible = true;
     }


     protected override void FollowTarget(float deltaTime)
     {
         if (m_Target == null) return;

         // Move the rig towards target position.
         transform.position = Vector3.Lerp(transform.position, m_Target.position, deltaTime*m_MoveSpeed);
     }


     private void HandleRotationMovement()
     {
         if(Time.timeScale < float.Epsilon)
         return;

         // Read the user input
         var x = CrossPlatformInputManager.GetAxis("Mouse X");
         var y = CrossPlatformInputManager.GetAxis("Mouse Y");

         // Adjust the look angle by an amount proportional to the turn speed and horizontal input.
         m_LookAngle += x*m_TurnSpeed;

         // Rotate the rig (the root object) around Y axis only:
         m_TransformTargetRot = Quaternion.Euler(0f, m_LookAngle, 0f);

         // on platforms with a mouse, we adjust the current angle based on Y mouse input and turn speed
         m_TiltAngle -= y*m_TurnSpeed;
         // and make sure the new value is within the tilt range
         m_TiltAngle = Mathf.Clamp(m_TiltAngle, -m_TiltMin, m_TiltMax);
         

         // Tilt input around X is applied to the pivot (the child of this object)
         m_PivotTargetRot = Quaternion.Euler(m_TiltAngle, m_PivotEulers.y , m_PivotEulers.z);

         if (m_TurnSmoothing > 0)
         {
             m_Pivot.localRotation = Quaternion.Slerp(m_Pivot.localRotation, m_PivotTargetRot, m_TurnSmoothing * Time.deltaTime);
             transform.localRotation = Quaternion.Slerp(transform.localRotation, m_TransformTargetRot, m_TurnSmoothing * Time.deltaTime);
         }
         else
         {
             m_Pivot.localRotation = m_PivotTargetRot;
             transform.localRotation = m_TransformTargetRot;
         }
     }
 }
 }