NOT THE COSTCO ONE - it's FOUL.
That out of the way, I do as everyone else here - the Ikea one (this year you buy all the bits separately, so I'm going to get my own sweeties, and make my own icing) - which I put together, then they decorate. Then 2 days later they lever all the sweets off and eat them, and DP and I get the gingerbread.