AddChild

using UnityEngine;
using UnityEngine;
using UnityEditor;
using System.Collections;

public class AddChild : ScriptableObject
{

    [MenuItem ("GameObject/+Add Child")]
    static void MenuAddChild()
    {
        Transform[] transforms = Selection.GetTransforms(SelectionMode.TopLevel | SelectionMode.OnlyUserModifiable);

        foreach(Transform transform in transforms)
        {
            GameObject newChild = new GameObject("_null");
            newChild.transform.parent = transform;
        newChild.transform.localPosition = Vector3.zero;
        }

    }

}