i think most of the fairs just happened for the holiday season....there's always the pokfulham open-air market that happens twice a month. it's very child friendly with large open spaces and colored plastic balls everywhere.
The range of products at the Pok Fu Lam market is pretty poor though. I stopped going as feel they have really gone down hill - the last one I attended really felt like a jumble sale.
Yes, I would not go for the actual items at the fair, but we found it great to let our son run around in an open place with grass. There are tons of families.