Create nested Child Gameobject in selected transform.

Child.cs

using UnityEngine;
using UnityEditor;
using System.Collections;

public class Child : MonoBehaviour {

  [MenuItem ("GameObject/Create Child #&n")]
	static void DoSomething () {
		if (Selection.activeTransform){
			var go = new GameObject();
			go.name = "Child";
			go.transform.parent = Selection.activeTransform;
			go.transform.localPosition = Vector3.zero;
			go.transform.localRotation = Quaternion.identity;
			go.transform.localScale = new Vector3(1,1,1);
		}	
	}
}