ADD water to drink mix in large plastic or glass pitcher; stir until drink mix is completely dissolved. Stir in grape juice.
USE immediately or refrigerate until ready to serve.
POUR 3/4 cup of the prepared drink mix into each of 8 tall glasses. Add 1/4 cup seltzer to each glass, then top with 1/4 cup ice cream. Serve immediately. Store leftover prepared drink in refrigerator.