Download serpent template at parents.com/sea-serpent. To craft the top, cut scalloped strips of green felt and fabric to fit width of shirt. Hot-glue strips to shirt, starting at the bottom and making sure rows overlap, alternating colors and textures.
Hat: Cut out 2"-wide circles of green felt and hot-glue to brim and cap, overlapping them to create a scaly look. Cut out and glue on white and red felt eyes to either side of hat (so they will sit above child's ears). Cut dorsal fin from green felt and hot-glue to center of hat. Cut a forked tongue from red felt and glue to front edge of brim.
Tail: Cut one leg off the green tights and stuff with batting until it is fully plump. Cut a fan shape from green felt and hot-glue to tail's closed (toe) end. Cut a 5'-long section of green felt with a scalloped edge on one side, then wrap around the tail approximately six times; hot-glue to attach. Seal the larger end of the tail shut with hot glue and glue to a length of green ribbon. Tie ribbon around your child's waist.