After the flower is smashed, remove the plastic bag and scrape off the flower remnants. When you're finished, you should see a distinct impression of the flower on the fabric. Experiment with different types of flowers and petals for a variety of colors. Let your child place them in a random pattern and smash them with the hammer. When your child is satisfied with the number of flowers she's smashed, frame the fabric.
Embroidery hoops work well since they automatically stretch and frame the fabric.
Hang the finished creation on the wall so that you and your child can enjoy the flowers all year long.