Like the Toy? See the Movie.
LEAD: You ordered the burger, the fries and the Coke, but you certainly didn't order the Flounder. There he is anyway: a three-inch blue-and-yellow rubber replica of a character from ''The Little Mermaid'' right there in your McDonald's Happy Meal, in a box adorned with jokes and puzzles that test your knowledge of Little Mermaid lore.