Spoon icing into a zip-top plastic bag; seal. Snip a 1/4-inch hole in 1 corner of bag. Pipe 2 rows of points around 1 cone, beginning at large end and working upward, to resemble a tree. Sprinkle with candy and, if desired, glitter. Repeat procedure until cone is decorated. Invert 2 cones; stack, securing with icing. Repeat decorating procedure. Invert remaining 3 cones, and stack, securing with icing; repeat decorating procedure. Insert larger candy pieces into trees, if desired. Let stand 8 hours. Store in a cool, dry place up to 1 month.
NOTE: For fuller trees, double icing recipe, and repeat piping procedure.
For the kids: Cut marshmallows from 1 (16 ounce) package in half with powdered sugar-coated scissors; cut each half into 4 pieces. Spread cones with 1 (16 ounce) container ready-to-spread cream cheese frosting. Press marshmallow pieces in rows around cones according to directions, adding frosting as necessary. Insert candy into icing between marshmallows; lightly rub additional icing on marshmallow leaves. Sprinkle with candy and glitter.