How can I find a specific child gameObject with multiple instances?

During the game, a prefab gameObject "XParent" is Instantiated multiple times, each with 1 child object called "Cube",

I have added a script to the "XParent" gameObject prefab. When the player collides with "XParent", the texture of the child gameObject, "Cube" should change but I am unable to target the specific child of the prefab, because there are multiple instances of "Cube" within the game.

I can easily target a specific transform component of XParent child using...

 for(var child : Transform in transform)

However, because there are multiple gameObjects called "Cube" when I use...

 var xRenderer = gameObject.Find("Cube").GetComponent(Renderer);
 xRenderer.material = xMaterial;

...Unity doesn't know which instance of Cube to target.

If only the following script was legal, my problem would be solved.

 for(var child : GameObject in transform)
 child.material = xMaterial;

I have searched on line for about 4 hours with no luck. Pleas help me target a specific child gameObject of a parent that the script is attched to.

You almost got it ... try this:

for(var child : Transform in transform)
{
    child.renderer.material = xMaterial;
}

If you don't have a reference to the cube, what yoyo said is fastest, unless the parent doesn't have a renderer(*). You should be aware of Transform.Find, though.

var xRenderer = transform.Find("Cube").renderer;

It wasn't necessary anyway, but don't use that form of GetComponent, that you did, anymore; it's slow. This is better:

var xRenderer = transform.Find("Cube").GetComponent.<Renderer>();

(*) In that case, use GetComponentInChildren.