You can start from scratch or you can cheat a bit like me and buy a cheap, plain white frosted cake so that everyone has the same cake from the grocery stores. No baking! That way your time is spent on being creative instead of mixing and cleaning. The purpose of this is activity is the decoration, not the making of the cake.
Buy mixed frosting colors for each cake, then have a number drawing to see who picks their frosting color first. By using an assortment of frosting color the results of the creations will be unique. Host a decorating contest among your own family or with some of the neighbor kids. Each person gets to take home their creation so they can share those tasty calories with their own family. This idea also makes a unique date activity! Your own version of Cake wars.
Mini cookies like animal crackers
Draw-on edible pens and frosting pens-fruit-more frosting
The Dollar stores have lots of decorations for... you guessed it, a dollar, that can be put on cakes. But remember to remind the kids that most are not edible. Like mini cars, trains, and doll figures.
A camera for all the pictures you are going to want to take
You can make it a social media contest if it is adults engaged in the contest. I think for young kids hoping for votes might be discouraging, but for adults it can be a lot of fun trying to get their friends to vote on their cake. Never say who decorated what cake though. Make the votes only on the cakes.