First Place: Clothes Pin. Use a clothes pin and attach it to the bridge. You can even use two or you could use something like a quarter to add mass to mute it even more. Just put the dime/quarter/penny on the bridge and clamp it on with the clothes pin.
Second Place: 2 Magnets. Use two magnets, one on either side of the bridge
See pictures and discussion here: https://www.fiddlehangout.com/archive/31030
Third Place: Eraser. This one is third place because it is hard to make this mute without breaking the eraser. Get an adult to help you cut an eraser as shown here: https://www.instructables.com/id/Violin-Mute/