Displaying Only Child Categories of Multiple Parents

This is one I have looked up and down for, but for whatever reason, just can’t seem to figure out.

I currently have a category setup like this:
Parent A
– Child A
— Sub-Child A1
— Sub-Child A2
– Child B
— Sub Child B1
— Sub Child B2
Parent B
…

What I really wanna do is just list the Sub-Child categories of Child A, Child B, … from the parent of Parent A all in a row. The end result would be something like…

– Sub-Child A1
– Sub-Child A2
– Sub-Child B1
– Sub-Child B2

I assumed this would have to be done using the new wp_list_categories tag, but I’m just not seeing the syntax to use, or would a custom query need to be used instead? Any assistance and/or thoughts and ideas would be appreciated. Thanks!