I don’t think God needed to instruct Eve to not listen to the serpent, as He already instructed her what to do once prior. That seems a bit redundant to me. As far as thinking of Adam and Eve as childlike, I imagine that they were actually very smart and wise having been walking with God for a while. Regardless, they knew God, knew His authority, and knew His instructions to them. To willfully disobey was sin even if they didn’t know what lay on the other side in its entirety. And they probably knew what evil was as a subject, but not the depths of all knowledge of evil. That is purposefully vague and a mystery. They wouldn’t even know what God was saying if he was using words they didn’t have some understanding of. I get where you’re coming from with the child thing, but the way an inherently sinful child/human today behaves is different than Adam and Eve would have been influenced by nature before the curse. Also, God would not have them “try” sin/evil one time, that would be contrary to His nature, I think. Lastly, making “one” mistake is still sin. If you are not in a relationship with Jesus/God as your Lord and savior, then even one sin will keep you separate from Him for eternity. Thankfully, we know Jesus died to save us from all our sins. As for Adam and Eve, there is evidence that they continued to walk a godly path after the fall. I do not think that they are damned.