Thanks for the response, Platonides.
I have to admit I can't figure out how that page is to prove that
subpages are taken into account in a ToC.
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'll give it another try on our own wiki tomorrow though.
-- F.