How to get the global/world position of a child object?
You can extract the world position like so:
var target = new THREE.Vector3(); // create once an reuse it
child.getWorldPosition( target );
target
will contain the result.
EDIT: updated to three.js r.120
In threejs r89 you can just get the world position by Object3D.getWorldPosition
. I don't know when it was added.