GameObjectExtensions.cs

using UnityEngine;
using System.Collections.Generic;

public static class GameObjectExtensions {
  
    public static T[] GetComponentsInChildrenOfAsset<T>( this GameObject go ) where T : Component {
        List<Transform> tfs = new List<Transform>();
        CollectChildren( tfs, go.transform );
        List<T> all = new List<T>();
        for (int i = 0; i < tfs.Count; i++)
            all.AddRange( tfs[i].gameObject.GetComponents<T>() );
        return all.ToArray();
    }
   
    static void CollectChildren( List<Transform> transforms, Transform tf){
        transforms.Add(tf);
        foreach(Transform child in tf){
            CollectChildren(transforms, child);
        }
    }

}