CoinManager.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CoinManager : MonoBehaviour
{

    public GameObject Door;

    void Start()
    {
        
    }

    void Update()
    {
        if(transform.childCount <= 0)
        {
            Destroy(Door);
            Destroy(gameObject);
        }
    }
}