We do this for Sukkot, the holiday of booth building. The kids get to build their own out of graham crackers and icing and candy. The green licorice is a nice substitute for leaves. You could adapt this for Christmas or whatever else you want.
My children can't believe I'm letting them handle all this sugar, but it's a holiday, and I am not a grinch.
The object of course is to figure out how to pile as much candy as you possibly can onto you booth, because you will get to eat it all. After dinner.
You will need,
Candy (licorice tape, colored balls, whatever you want really)
I don't usually bother with making royal icing with the egg whites. It is the strongest by far but this recipe works just fine, tastes better, and holds things together well enough to look at for a while before eating.
Icing sugar, lots
1/4 lb butter
water less than 1/2 a cup
Cream together butter and sugar, add enough sugar so that the mixture is very dry. Slowly add a little bit of water until it reaches the consistency of a thick paste. If it's too thin, add more icing sugar. (This is very scientific.)
Then you can put it in ziplock bags and cut a little piece of the corner off to make and icing bag. Kids can squeeze their own icing out and use it to cement together the graham crackers into little boxes. For younger kids you can give them a small scoop of icing on their plate and they can dip the candies into it before gluing them to the sides top floor, whatever.
This is a lot of fun, you will want to jump in and make one yourself, and of course if you tell them that they can eat it after dinner for several days, they are less likely to gorge themselves all in one night. Mine don't ask for candy again for a looong time. They usually feel ill, which is fine with me, it may help them learn moderation.