Place individual scoops of ice cream into the pie crust, forming a mountain of ice cream (with the center scoop as the highest point). Drizzle with hot fudge sauce and sprinkle with walnuts. Place animal crackers on top of each scoop and serve immediately. If not planning to serve right away, then place the ice cream scoops into the crust, cover loosely, and store in the freezer. Just before serving, drizzle with hot fudge sauce, sprinkle with walnuts, and place the animal crackers on each scoop.