Rated 4 out of 5 stars
This is very close to exactly what I want. My main complaint is how it differentiated between parent and children, it would be nice if some options were given for it, personally I'd like to have two half height tab rows, the one on top showing parent tabs, the one on bottom showing children, then a set of up/down arrows to navigate depth. I'm fine working with the current system, but I find it unacceptable that expanding a parent tab increases the height of all top level tabs.
It would also be nice if there was a type of tab that exists only to contain tabs, and will display the last active child tab when selected.