Er... ToC takes into account the subpages headings http://en.wikipedia.org/w/index.php?title=Wikipedia:Sandbox&oldid=119738...
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