Grant permission only to children, not to parent

Hi,

I have a vocabulary like the following:

Parent1
---Child1
---Child2
Parent2
---Child3
---Child4

Using Taxonomy Access Control, I have been able to create a user role that permits users within that role to create nodes only with Parent1 tag.. And that includes Child1 and Child 2 as well.. which is fine for this user role.

Now I would like to create a user role which should have permission only to create nodes with tag Child1 (with no permission for Parent1 and Child2).

How do you think this can be achieved? If I wanted to customize it to achieve this, what do I need to do? Or, are there some modules out there that could help me in this direction?

I use: Drupal 6.15 (Pressflow) and MySQL.

Any help appreciated!

Thanks!