Instantiate as a child properly

So I found need to Instantiate a prefab as a child of object, but i'm not sure how it is done and I am always getting an error:

NullReferenceException: Object reference not set to an instance of an object Player_Statistics.Update () (at Assets/Scripts/Player_Statistics.cs:23)

Evolved form of Base_Player is created, but it is not a child of "Player". The script I have:

 using UnityEngine;
 using System.Collections;
 
 public class Player_Statistics : MonoBehaviour {
 
     public static int evolution_points;
     public int level;
     public Rigidbody2D evolution;
 
     // Use this for initialization
     void Start () {
         evolution_points = 0;
         level = 0;
     }
     void Update (){
         if (evolution_points >= 5) {
             evolution_points = 0;
             Debug.Log ("Level up!");
             foreach (Transform child in transform) {
                 GameObject.Destroy (child.gameObject);//destroy all childs
                 //creates evolved form of player at the same hierarhy possition
                 Transform evolved_player = Instantiate(evolution, child.position, child.rotation) as Transform;
                 evolved_player.parent = gameObject.transform;
                 return;
                  }
         }
     }
 }

Before playing game I have:

Please do not hesitate to give any tips, thanks in advance.