Ah! I've discovered that you have to specifically select the ">" in the mobile menu to be taken to the sub-pages. But at least for me (and several of my testers), that wasn't intuitive at all. If anyone has any ideas for tweaking that to make it clearer, I'd love to hear it.
For example, since I have on a few child pages, maybe they could render in the mobile menu with the main pages, just offset or aligned differently?