Dupal 6 Taxonomy coding

I have a Taxonomy hierarchy and need to be able to limit certain roles to editing only child terms, not the parent terms. I've looked through all of the modules that I can find that relate to this and haven't found any that fits this requirement, so it looks like I have to write a custom module. Any hints as to the direction I should be looking.

This is in Drupal 6.

Thanks in advance.