Prepare and bake cake as directed on package, using all cake ingredients and 13x9-inch pan. Cool as directed.
In small bowl, combine all frosting ingredients; beat until smooth. Frost cooled cake.
To make 3 flowers, unroll 1 fruit roll snack leaving backing attached. Cut snack into three 12-inch pieces. With scissors, make cuts 1/8 to 1/4 inch apart along 1 long side of each piece, cutting to about 1/8 inch from other long edge. Starting with short side, roll up each piece removing backing during rolling. Pinch uncut side of roll; fan "petals" to form flower. Repeat with remaining fruit roll snacks to make 9 more flowers. Arrange flowers on top of cake, pressing uncut side gently into cake. Use decorator frosting to form leaf shapes around flowers.