How to make a select from fixed vocab and add child free tags?

I can't seem to get this to work. I have a cck content taxonomy field with a drop select one for the parent. And I have a another field for the free tag child with the php to return the parent as

 return  $node->field_parent[0]['value']

It works if I return a fixed value so I guess it might be a weight issue but making the auto complete module heavier doesn't help. It would also be nice to be able to limit the auto complete to just those terms under the parent but auto complete seems to use all the vocab