How can I pull in the current page's children and their children based on the main menu, not actual site structure?

I've been at this for a while now and am looking for some help. I'm trying to build tabs (and put links within those tabs) based on the structure of a site's main menu which does NOT in this case correlate to the site structure.

e.g.

Page1

-Page1Child1

--Page1Child1Child1

--Page1Child1Child2

-Page1Child2

--Page1Child2Child1

--Page1Child2Child2

-Page1Child3

--Page1Child3Child1

--Page1Child3Child2

So Page1's children are not an actual pages, they are just links to an ID on Page1 (e.g. /page1/#tab_1) so the javascript on the page shows the proper tab.

I want to pull the page1 children in as the tab titles and their children in as links within the corresponding tab.

wp_list_pages() hasn't helped since the children aren't pages (the grandchildren are though) and wp_nav_menu() puts extra markup in the output that I can't figure out how to deal with. I've tried a custom walker as well and just can't get it to output anything useful (my fault, not it's fault).

Any ideas? Clear as mud?