How do I get my shared Google Calendars to show up?

You need to tell Google to sync all your sub-calendars to your iPhone: you can do that here: https://www.google.com/calendar/iphoneselect .

Exchange / Second option

If that doesn't work, please try the following:
  1. From your iPhone browser, go to https://www.google.com/calendar/syncselect
  2. Sign in with your account and you will be able to select calendars you want to sync with your iPhone.
  3. Please ensure that the language of Google's site is set to English for your iPhone
  4. Go back to the calendar application and you will have all your calendars you have selected.
source: Google