In a large pot, fully melt the butter over low heat.
Add the marshmallows and stir until completely melted, about 8 minutes.
Remove the pot from the heat and mix in the rice cereal.
Transfer the pot's contents onto the lined baking sheet. Use a flat surface to press the mixture into an even layer, about ¼ inch thick, on the prepared baking sheet.
Unroll 2 Fruit Roll-Ups. Join them together to create an elongated sheet by overlapping the edges and pinching. Repeat with the remaining Fruit Roll-Ups.
Cut out pieces of the pressed rice mixture to layer atop each Fruit Roll-Up, leaving a thin border of about ½ inch.
Add 4 gummy worms or Swedish Fish to the lower center of each rice cereal layer.
For each assembled sheet, roll tightly in an upward motion to create a long roll.