Array out of index, but shouldn't be

I have a grid that is 15 wide and 20 high, I also have a method in which checks and updates this grid when something comes into it and that object becomes apart of the grid, but the moment the object is instantiated the grid array becomes out of index... I've looked over it and it shouldn't, but as usual I'm most likely not seeing something. Here's the code:

 using System;
 using UnityEngine;
 using System.Collections;
 
 namespace GameManager
 {
     public class GameManager:MonoBehaviour
     {
         Instantiator instance;
         public static int gridWidth = 15;
         public static int gridHeight = 21;
         private static Transform[,] gridMatrix;
 
         void Start()
         {
             instance = FindObjectOfType<Instantiator>();
             gridMatrix = new Transform[gridWidth, gridHeight];
             instance.Spawn();
         }
         public void UpdateGrid(Tetro tetro)
         {
             for (int y = 0; y < gridHeight; y++)
             {
                 for (int x = 0; x < gridWidth; x++)
                 {
                     if (gridMatrix[x, y] != null)
                     {
                         if (gridMatrix[x, y].parent == tetro.transform)
                         {
                             gridMatrix[x, y] = null;
                         }
                     }
                 }
             }
             foreach (Transform mino in tetro.transform)
             {
                 Vector2 pos = RoundOffPositions(mino.position);
                 if (pos.y < gridHeight)
                 {
                     gridMatrix[(int)pos.x, (int)pos.y] = mino;
                 }
             }
         }
         public Transform getChildPositionInGrid(Vector2 pos)
         {
             if (pos.y > gridHeight - 1)
             {
                 return null;
             }
             else
             {
                 return gridMatrix[(int)pos.x, (int)pos.y];
             }
         }
         public bool CheckIsInGrid(Vector2 pos)
         {
             return (pos.x >= -7 && pos.x <= 7 && pos.y >= -10);
         }
         public Vector2 RoundOffPositions(Vector2 pos)
         {
             return new Vector2(Mathf.RoundToInt(pos.x), Mathf.RoundToInt(pos.y));
         }
         void Update()
         {
             
         }
     }
 }

and:

 using System;
 using UnityEngine;
 using System.Collections;
 
 public class Tetro:MonoBehaviour
 {
     Instantiator instance;
     GameManager.GameManager management;
     public bool hasLimitedRotation;
     
     void Start ()
     {
         instance = FindObjectOfType<Instantiator>();
         management = FindObjectOfType<GameManager.GameManager>();
         StartCoroutine(Fall());
     }
     IEnumerator Fall()
     {
         foreach(Transform mino in transform)
         {
             while (mino.position.y > -10)
             {
                 yield return new WaitForSeconds(1);
                 transform.position -= new Vector3(0, 1);
                 if (!checkEachChildPosition())
                 {
                     transform.position += new Vector3(0, 1);
                     instance.Spawn();
                     enabled = false;
                     yield break;
                 }
             }
         }
     }
     public void CheckInput()
     {
         if (transform.position.y > -9)
         {
             if (Input.GetKeyDown(KeyCode.RightArrow))
             {
                 transform.position += new Vector3(1, 0);
                 if (!checkEachChildPosition())
                 {
                     transform.position += new Vector3(-1, 0);
                 }
                 else
                 {
                     management.UpdateGrid(this);
                 }
             }
             else if (Input.GetKeyDown(KeyCode.LeftArrow))
             {
                 transform.position += new Vector3(-1, 0);
                 if (!checkEachChildPosition())
                 {
                     transform.position += new Vector3(1, 0);
                 }
                 else
                 {
                     management.UpdateGrid(this);
                 }
             }
             else if (Input.GetKeyDown(KeyCode.UpArrow))
             {
                 transform.Rotate(0, 0, 180);
                 if (hasLimitedRotation)
                 {
                     transform.Rotate(0, 0, -90);
                 }
             }
             else if (Input.GetKeyDown(KeyCode.DownArrow))
             {
                 transform.position -= new Vector3(0, 1);
                 if (!checkEachChildPosition())
                 {
                     transform.position += new Vector3(0, 1);
                 }
                 else
                 {
                     management.UpdateGrid(this);
                 }
             }
         }
     }
     bool checkEachChildPosition()
     {
         foreach (Transform mino in transform)
         {
             Vector2 pos = management.RoundOffPositions(mino.position);
             if (management.CheckIsInGrid(pos) == false)
             {
                 return false;
             }
             if (management.getChildPositionInGrid(pos) != null && management.getChildPositionInGrid(pos).parent != transform)
             {
                 return false;
             }
         }
         return true;
     }
     void Update ()
     {
         CheckInput();
     }
 }

this is the error on the stack:

 IndexOutOfRangeException: Array index is out of range.
 GameManager.GameManager.getChildPositionInGrid (Vector2 pos) (at Assets/Scripts/GameManagers/GameManager.cs:52)
 Tetro.checkEachChildPosition () (at Assets/Scripts/Tetros/Tetro.cs:94)
 Tetro+<Fall>c__Iterator0.MoveNext () (at Assets/Scripts/Tetros/Tetro.cs:25)
 UnityEngine.SetupCoroutine.InvokeMoveNext (IEnumerator enumerator, IntPtr returnValueAddress) (at C:/buildslave/unity/build/Runtime/Export/Coroutines.cs:17)