TileMap inside a Tilemap

How do I make a TileMap inside another Tilemap the correct way? The main tile map has cubes which are 10x1x10 and some of the tiles have another 10x10 tile map inside them, but not 1 cube. The script I have right now, but it doesn't work as I want it to:

 public class TileMapGenerator : MonoBehaviour {
 
     public static TileMapGenerator instance;
 
     public GameObject[,][,] tiles;
     public GameObject[,] fields;
 
     public GameObject fieldPrefab;
     public GameObject tilePrefab;
 
     public int worldMapSize;
     public int tileMapSize;
     public int fieldSize;
     public int tileSize;
     public int centerPos = 5;
 
     public Material[] materials;
 
     void Awake() {
         if (instance == null) {
             instance = this;
         }
         GenerateTileMap ();
     }
 
     private void GenerateTileMap() {
         fields = new GameObject[worldMapSize, worldMapSize];
         tiles = new GameObject[worldMapSize, worldMapSize][,];
         for (int x = 0; x < worldMapSize; x++) {
             for (int y = 0; y < worldMapSize; y++) {
                 fields [x, y] = new GameObject ("Field");
                 fields [x, y].transform.localPosition = new Vector3 (x * fieldSize + fieldSize/2, 0f, y * fieldSize + fieldSize/2);
                 fields [x, y].transform.SetParent (transform);
                 if (x == 5 && y == 5) {
                     GenerateTiles (x, y);
                 }
                 GameObject newField = BuildTile (10f, "Tile");
                 //newField.transform.SetParent (fields[x, y].transform);
                 newField.transform.position = new Vector3(x * fieldSize, 0f, y * fieldSize);
                 newField.transform.SetParent (fields[x, y].transform);
             }
         }
     }
 
     private GameObject BuildTile(float size, string name) {
         GameObject newTile = GameObject.CreatePrimitive (PrimitiveType.Cube);
         newTile.name = name;
         newTile.transform.localScale = new Vector3 (size, 1f, size);
     
         MeshRenderer rend = newTile.GetComponent<MeshRenderer> ();
 
         return newTile;
     }
 
     private void GenerateTiles(int k, int j) {
         tiles[k, j] = new GameObject[tileMapSize, tileMapSize];
         for (int x = 0; x < 10; x++) {
             for (int y = 0; y < 10; y++) {    
                 tiles [k, j][x, y] = BuildTile (1f, "Tile");
                 tiles [k, j] [x, y].name += "_" + x + y;
 
                 float posX = x - fieldSize / 2f;
                 float posY = y - fieldSize / 2f;
                 Vector3 pos = new Vector3 (posX, 0f, posY);
 
                 tiles [k, j][x, y].transform.position = pos;
                 tiles [k, j] [x, y].transform.SetParent (fields[k, j].transform);
             }
         }
     }
 
 }