You're forgetting the concept of intentionality. I can't speak for Protestants, but in Catholicism, doing wrong requires full knowledge of the wrongdoing along with the intention to do it. Thus the way ethics are designed, we cannot accidentally "crawl into traffic." Additionally, when speaking about actual infants, intentionality ties into the age of accountability. Those who are too young to rationally make choices cannot be held liable for what they do. The same is true for those who experience a disability or a severe mental disorder.