GameObjectWithTag Child

Good question and upvoted! I made a sample script that solves the issue hopefully.

 #pragma strict
 #pragma downcast

 var parent : Transform;

 function Start () {
     var childObject : GameObject = FindChildGameObjectWithTag(parent, "MyTag");
     if (childObject) Debug.Log(childObject.name+" was found with your tag");
 }

 function FindChildGameObjectWithTag (p : Transform, tagName : String) : GameObject {
     for (var child : Transform in p) {
         if (child.tag==tagName) {
             return child.gameObject;
         }
     }
     return null;
 }