Microsites in WordPress

Hi,
I am trying to build a website that has broad division of segments in the main menu. It could be something like “Healthcare”, “Personal Finance” etc. Then each of these segments further divide into categories and pages. Each of these segments can have slightly modified CSS and header graphics.

Now, my problem is that I’m stuck. I’ve laid out the top level categories, but the sub navigations refuse to show up, and I get a message like “No categories.” (I used a WP function to fetch children of a specified catid.)

Can I find some plug-in for that? I googled it out but I got no good results. Somebody please help!