If it's attached to the parent you can address a specific gameobject using either way:

1.If children are already there before running the game you can simply define a gameobject variable and assign the child to it.

var top:GameObject;

top.renderer.material.mainTexture = ....;

2.Use this way if your children are dynamically being added or removed and also if you know their names.

var top:GameObject = transform.Find("top").gameObject;
top.renderer.material.mainTexture = ....;