Neither, but more on the kid's side. If the child were an adult that would be considered assault had it caused hearing loss in the victim. However the adult, autistic or not, intentionally struck the child with the intention of causing harm and therefore will be arrested and charged with assault/battery (depending on the state).
People with cognitive disabilities don't deserve any slack. Especially people with autism. They damn well know right from wrong. I know this first hand due to having worked with some and going to school with some. They know damn well what is right and wrong. And they are capable of learning when you are stern with them. They just can't analyze social behavior such as facial expressions, tones of the voice, when to shut up, etc..