Definitely Sydney! A really fun city and lots of things you can do with your kids like the Taronga Zoo or Luna Park! I would look and see when the Sydney festival is, could be another fun option for all of you.
Alica Taronga Zoo is much better! Actually, I belong to Aussie Campervans (facebook.com/Aussiecampervan) but one of visitors asking for the same which I questioned here. Because, in Trippy, people belong to Australia will make me to answer the truth on the basis of other's experience.