Place the watermelon juice in an 8-by-8-inch glass baking dish and freeze. After 40 minutes, using a fork, scrape the watermelon ice to create a granita. Place some of the granita in each of 4 chilled soup plates. Place 4 watermelon cubes in each plate. Add the lemon, orange, mint, edible flowers and season with salt and pepper. Just before serving, dust each plate with pop rocks.