Intermediate Needs SQL HELP

I’m new to wordpress and I need some database help and maybe some pseudo code. Consider me a layman…

HISTORY
I have purchased a theme https://demos.appthemes.com/?theme=classipress and I’m trying to change the theme’s “Category” dropdown. I’ve been in contact with the developers, but to no avail.

What I Want To Do
I want to modify the themes primary navigation to display only the categories each with their own seperate dropdown menu that displays the children of said categories. The native function that comes with this theme will not allow this so I need to write my own hack. Only, I do not understand the relationships between wp_terms, wp_relationships, wp_taxonomy and wp_posts.

What I Think I Need To Do:
1. Understand the database a little more ??
2. Write a custom function that takes a couple parameters to select the parent-category based on parameter and display in a drop down only the sub-categories. Sounds easy, but this database is what throws me off…
3. The displayed sub-categories will of course be links that will then display the associated data.

There is a deadline to be met so I do not care if I have to write a custom hack that is placed in the presentation layer. Also this is not a big applications so I’m not worried about resources.

Can you help?