Script messing with Draw Order?

Hi All

I have a script that is applied to parent objects that basically changes the shader of all children to a transparent/diffuse, and links up the alpha value to a slider in another script.

 function Update () {
   for (var child : Transform in transform) {   
     child.renderer.material.shader = Shader.Find("Transparent/Diffuse");
     child.renderer.material.color.a = TransparencyGlobalSlider.SliderValue;

}

}

It works as intended, however there is one strange side effect.

Basically it appears to mess with the draw order - any parent that has this script applied to it, the child objects become visible through other objects.

So for instance I have a road object, and a bridge object underneath. If I apply this script to both, suddenly the bridge is able to be seen when it should actually be under the road.

Any idea why this would happen?