I had this problem. Google do have an answer, but it's not the most obvious.
Assuming you have set up your calendar according to the instructions found here:
http://www.google.com/support/mobile/bin/answer.py?answer=138740&ctx=sibling, then you need to go to the following address in Safari: m.google.com/sync
Select 'iPhone' and then tick the calendars you wish to sync with your phone.
Hope that works