For the crust and filling: Spray a 9-inch pie pan with nonstick cooking spray.
Melt the butter in a medium pot over medium-low heat. Add the marshmallows and cook, stirring, until melted. Add the fruity cereal and mix until completely coated. Transfer to the prepared pie pan and press the mixture into the pan to form a crust. Let cool completely.
Spread the ice cream over the crust and smooth the top with a spatula. Freeze until hardened, 4 to 6 hours.
For the garnish: In a chilled large metal bowl, combine the heavy cream, sugar and vanilla. Whip the cream with a whisk or hand mixer to medium peaks.
Vigorously shake the shell topping sauce and pour about 1/2 cup over the ice cream. Let set. Using a 2-inch ice cream scoop, place scoops of the whipped cream around the edge of the pie. Sprinkle each whipped cream scoop with rainbow sprinkles. Hold in the freezer until ready to serve.