One can't be guilty of "sin" when one doesn't have knowledge of the difference between good and evil. Nor does one have "free-will" in such a circumstance.
Only by gaining the knowledge of good and evil did human beings, in a mythological sense, become moral beings and (as acknowledged in the story) more like the divine.
Not following an order (disobedience) that one doesn't understand on a moral level isn't "sin." To sin one must have moral discernment. That moral discernment was completely lacking in the story prior to the eating of the apple. Gaining that ability was what made people fully human on a mythic level.
Otherwise, we were like dogs or babies, beings whose "disobedience" carries no moral weight (since they don't know good from evil.
I disagree that they lacked moral discernment. Why would God even tell them not to eat from that particular tree if they lacked the capacity to obey?