We love toilet paper roll crafts - they are frugal and fun and the possibilities are virtually endless (I call all these crafts TP roll crafts although we often use kitchen towel rolls too - also great for those that don't like using TP rolls).
First you have to color the paper roll black (I like to color them on the inside too as they look better in the end - just my personal thing). We usually color them with sponge brushes as it's super easy, quick and the paint is always applied evenly (not that anything is wrong with not evenly ha! - I was an everything must be even freak as a kid).
Next you have to flatten the TP roll a bit so that it is easier to cut and cut the fish shape as seen on the picture. Depending on kids age and scissor skills this is the part you may step in.
Last but not least - drawing the fish bone and let it dry. Sponge brush won't be of use here, you will need your plain brush.