Unroll fruit rolls, and place, with plastic sheet down, on cutting board.
Melt butter in a Dutch oven over medium heat. Add marshmallows, stirring until melted; remove from heat. Stir in rice cereal until blended.
Spread about 1/2 cup cereal mixture quickly over each fruit roll, leaving a 1-inch border on 1 long side. Arrange 2 worm-shaped candies or 3 fish-shaped candies lengthwise down center of cereal mixture.
Roll up, starting at side without border. Press to seal securely. Repeat procedure with remaining fruit rolls, cereal mixture, and candies.
Cut each roll into 4 slices. Serve with chopsticks, if desired.