SPRINKLE gelatine over 1 cup of the root beer in medium saucepan; let stand 1 min. Cook on low heat 5 min. or until gelatine is completely dissolved, stirring frequently. Remove from heat; cool slightly.
ADD 1 cup of the whipped topping; stir with wire whisk until well blended. Add remaining 2-1/2 cups root beer; stir. Pour into 6 (8-oz.) parfait glasses, distributing foam evenly among glasses.
REFRIGERATE 2 hours or until set. Top evenly with the remaining 1/2 cup whipped topping and the cherries just before serving.