You can bake ahead and freeze the unfrosted cake (tightly wrapped) for up to 2 months.
If you don't have enough pans to bake all the layers at one time, cover and refrigerate the rest of the batter while baking the first batch. Cool the pan(s) about 15 minutes, then bake the rest of the batter, adding 1 to 2 minutes to the bake time.
Don’t have 6-inch round pans? Use four 8- or 9-inch round pans. Divide batter evenly among pans, and bake as directed on box. Cut 2 of the cooled cake layers to 6 inches.
Rock candy sprinkles can be found online or at a store that specializes in crafts and decorating supplies.