Trim the tops of all the cakes to make them level. Place a 9-inch cake on a serving platter and cover the top with vanilla frosting. Stack the other 9-inch cake on it and spread a thin layer of frosting on top. Refrigerate the cakes for at least 30 minutes. Prep the two 6-inch cakes in the same way.
Cover each chilled two-layer cake with vanilla frosting. Place the smaller cake, on the cardboard, on the larger cake. With the chocolate frosting, pipe a birthday message onto a large lollipop, then insert it into the top of the cake. Embellish the rest of the cake with smaller lollipops by inserting the sticks into the cake.
To transfer the small cakes more easily later, put one layer on a 6-inch-wide foil-covered circle of cardboard, then add the second layer.