Would there be any possibility of making it with cloth instead of clay? Probably the same concept, foil and wire mesh inside for support, but maybe use material and sequins for scales instead of clay? Maybe even some cotton batting in between to help protect the inner structure from getting misshapen? I've never done a merpony at all, so I dunno how doable it would be, but for that age I think I'd try to avoid clay altogether; it's a lot of work to risk being broken.