Keep the other balloon out. After a whole day, take the balloon out of the freezer. Hold it up next to the other balloon. The balloon in the freezer got smaller. The air inside the balloon got colder.
Cold air takes up less space than warm air.
Keep the cold balloon near the other balloon. Soon it will be big again. Soon it will be as big as the other balloon. Ask them to draw a picture of what they think will happen to the balloon. Look at the air intake hole where air comes in to be pushed out by the pump.