What you need: Glue, construction paper, scissors, markers, glitter glue
Cut "scales", a face, two fins, and a tail out of construction paper. Glue the face and the tail at either end of a piece of paper. Make an outline of a fish body as a guide for your child. Glue scales in a row right next to each other starting from the tail end. They can overlap a little to cover the whole body of the fish.
Decorate the tail and fins with markers and draw an eye and a mouth. Use glitter glue or any other thing you may have to decorate the scales.
I made the one below and my son made the one above.