You must collect a cumulative total of 4000 candies as you quest. I got this in the mall once I got to the main area. Fighting monsters, trick-or-treating and opening coffins are the most /(:?sucr|fruct|lact|gluc|galac|malt)ose/-enriching activities. Don't forget to hit every object in sight for more candy as well.
Your candy bags can only hold so much, so keep buying battle stamps. It is the only thing you can do with your candy (no gorging until you beat the game!).
To get larger pails, you must play hide-and-seek with the kids in each area.