Collision with avatars and scene not working

[JvdBosch]

OK, so the problem is calling a collidemesh on a child. Knowing this I tried to detach the children I need to collide with from the mall object with Object.parent(viz.WORLD), but they just disappear. Apparently viz.World is not the current scene. How do I detach children from this node and attach them to the main scene? I tried viz.SCENE and viz.HEAD and viz.MAIN_SCENE, but none work)
I don't want to collide with the whole mall object, because it's rather large and I just need to collide with 14 objects to have the avatars walking in the right places.

The sky for example is a child of the main scene (it's added with viz.add), but when I request the parent of the sky with sky.getParents() it's empty... So how to attach a child to the main?