I'm looking at some local bakery's websites and as I'm going through the birthday cakes I see they have a section for Birthday Kit Cakes. What is a kit cake? They don't really explain on the site. The prices seem pretty cheap.
Around here, they used to call them "set" cakes because it's a set of plastic items to decorate your cake with or a kit that you purchase to set the plastic items upon your decorated cake.
DecoPac is one company who puts out such 'kits'/sets.
Check out their website.
Here are a couple I made: