The initial velocity of the ball when the child lets it go at the top of the driveway is zero.

Here's why:

* Initial Velocity: This refers to the velocity of an object at the very beginning of its motion.

* At Rest: When the child holds the ball at the top of the driveway, the ball is not moving. It's at rest.

* Zero Velocity: A velocity of zero means the object is not moving.

Important Note: This assumes the child simply lets go of the ball without giving it any initial push or throw. If the child throws the ball, then the initial velocity would be whatever speed the child imparts to it.