Actually, I'm not even sure it would be manslaughter. With involuntary manslaughter one has to be aware that the situation that caused the death was dangerous. So just hitting someone with your car wouldn't automatically be manslaughter, but it would be if you were drinking, ran a red light, or doing something else considered reckless or negligent. If she truly believes she's hitting a piñata, you might have a hard time proving negligence on her part.