I saw a really cute cake online. I would use 2 bar cakes and carve a head and body out of one and a tail and feet out of the other. Cover with green fondant, texture with a ball tool by making slight curved impressions to represent scales. Use the ball tool to make impressions for the eye sockets and the nostrils, use a leaf vainer to draw a mouth and separate the toes. Add scales along the back from the head to tail tip - circles of fondant cut in half in graduating sizes large to small. Use white royal icing to pipe claws and teeth and make eyes from small fondant circles in white and black. Decorate the cake board with part blended green, brown and blue fondant and add fondant lilly pads, maybe a flower or two and some blades of swamp grass - cut strips of gumpaste, shape into blades of grass and dry before pressing into small brown balls of fondant on the board - use flower wire and brown gumpaste to make duckweed flowers.
This is from the book Hello Cupcake. Very easy and cupcakes are always a hit with kids. Good luck with whatever you decide.