Here's my number one rule: Buy a kit, preferably one with a preassembled house. When you use a kit, you spend all your time on decorating the house, which is the fun part. Building it is a job for an adult, and you have to let the icing harden before you can add the candy. This requires patience, which kids do not have a lot of. (I'm not even going to touch the idea of baking your own gingerbread. If that's where you want to start this project, maybe Martha Stewart can help you. Or maybe several years of therapy.)
Check out the candy selection before you get started on the project. The more variety, the better. If you need to, head to Walgreens and find some small, brightly colored candies to add to what came with the house.
Be prepared for a mess. It's shouldn't be a huge mess, but there will be candy bits, stray icing, and gumdrop sugar scattered on the table and floor. Everyone will have more fun if they don't have to worry about being neat while they're trying to create! Be sure to factor in some clean-up time.
Don't be a perfectionist. Keep this activity about the "doing" and not about the end result. Your gingerbread house might not be ready for a magazine cover shoot, but if the project is something that your family has fun doing together, you will all love it. Put on some Christmas tunes and have fun with it. If you happen to be an artist or something, and you really want a final product that's a showpiece, then you should make another one as your own personal project.
Consider decorating more than one. If your family really gets into this, you could let everyone decorate their own house. Kids might like to have their house as a Christmas decoration in their room. Or you could give extras as gifts. Grandparents will love these. Maybe even the piano teacher. (If you're going to give them away, be sure to do it early in December so there will be plenty of time to enjoy it.)
Don't keep it past the holiday season. When you put away the other Christmas decorations, it's time to throw out the gingerbread house. If you want to be green about it, you can add it to the compost pile.
Even if gingerbread houses aren't your thing, holiday traditions are a fun way to bring the family together. It's a much better way to celebrate the season than shopping!