That shows transcluded pages, not subpages.
So I've put up a test on Wikia's Scratchpad
(MW v1.9.3):
http://scratchpad.wikia.com/wiki/Sandbox/Subpage_lvl1
http://scratchpad.wikia.com/wiki/Sandbox/Subpage_lvl1/Subpage_lvl2
Look at the ToC on the first page; it does not include the headings from
the level 2 page.
I think the best you can get is to an automated list of the subpages,
not the TOC from them.
{{Special:Prefixindex/Sandbox/Subpage lvl1/}}
Angela