As George and Betty drive off, both in the front seat of Betty's car, the back door opens and Tony jumps in. You can see the hand of a stage hand on the window of the car door holding it steady for Bixby.
When Nicholson's grandson is tossing the ball up and down, it's clearly a rubber ball and it can be heard "smacking" into the boy's hand. But when Tony "borrows" it, it turns into a sponge ball and the boy never notices or comments on it.