How can i expand/collapse all childs under a gameobject ?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class InstantiateObjects : MonoBehaviour
 {
     public GameObject prefab;
     public Terrain terrain;
     public float yOffset = 0.5f;
     public int objectsToInstantiate;
     public bool parent = true;
 
     private float terrainWidth;
     private float terrainLength;
 
     private float xTerrainPos;
     private float zTerrainPos;
 
 
     void Start()
     {
         //Get terrain size
         terrainWidth = terrain.terrainData.size.x;
         terrainLength = terrain.terrainData.size.z;
 
         //Get terrain position
         xTerrainPos = terrain.transform.position.x;
         zTerrainPos = terrain.transform.position.z;
 
         //generateObjectOnTerrain();
     }
 
     public void generateObjectOnTerrain()
     {
         for (int i = 0; i < objectsToInstantiate; i++)
         {
             //Generate random x,z,y position on the terrain
             float randX = UnityEngine.Random.Range(xTerrainPos, xTerrainPos + terrainWidth);
             float randZ = UnityEngine.Random.Range(zTerrainPos, zTerrainPos + terrainLength);
             float yVal = Terrain.activeTerrain.SampleHeight(new Vector3(randX, 0, randZ));
 
             //Apply Offset if needed
             yVal = yVal + yOffset;
 
             //Generate the Prefab on the generated position        
             GameObject objInstance = Instantiate(prefab, new Vector3(randX, yVal, randZ), Quaternion.identity);
             if (parent)
                 objInstance.transform.parent = this.transform;
         }
     }
 }
 

When they are parent they are under a GameObject as childs. How can i do in script that when running the game in the hierarchy it will expand the childs or collapse ?

This is when they are all show:

And this is unexpanded