Tint 3 tablespoons of frosting with yellow food coloring, spoon it into a ziplock bag, and snip off a corner. Tint the remaining frosting purple. Trim the top of one of the cakes to make it level. Set the cake on a sheet of parchment paper to catch extra sugar sprinkles. Cover the top of the cake with a layer of purple frosting, then place the other cake on top. Create the look of a tufted pillow by using a serrated knife to make an X-shaped indentation across the cake, corner to corner (the lines will serve as a guide for the stitching). Cover the cake with the remaining purple frosting, then coat it with purple sparkling sugar. Transfer the cake, on the cardboard, to a serving platter. Return any excess sugar to its container.
Press a yellow gumdrop, flat side up, into the center of the cake. For the tassels, halve two yellow fruit twists widthwise, then cut each piece into fringe, leaving a 3/4-inch uncut. Bore a 1/2-inch-deep well in the bottom of four more gumdrops and press a pretzel stick into the top. Add a fruit twist fringe to each as shown. Pipe yellow frosting stitching along the cake's edges and in the X. Just before serving, insert a tassel into each corner and set a plastic tiara or crown on top.
To make it easier to transfer the cake later on, put it on a 9-inch square piece of foil-covered cardboard.