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;
}