STIR boiling water into dry gelatin mix in large bowl at least 2 min. until completely dissolved. Stir in club soda. Refrigerate 1-1/2 hours or until thickened (spoon drawn through leaves definite impression).
ADD melon balls to gelatin; stir. Spoon into eight champagne glasses or dessert dishes.
REFRIGERATE 3 hours or until firm. Top with dollops of the whipped topping just before serving. Store any leftover desserts in refrigerator.