They wouldn’t know good and evil because they wouldn’t know evil. There’s no inherent need for evil in God’s creation. The first evil they experienced BTW was their own sin, which immediately put them in a compromised or damaged position vis a vis God and their relationship with Him. They had effectively rejected His Godhood, and this act/state was itself a matter of injustice, casting them into a very different sort of world where a rift now existed, between themselves and God, themselves and their fellow man, themselves and creation, and between and within their very own selves.
And while innocent, they weren’t ignorant of God’s command. That command was written in their hearts/consciences already. It was like the command against murder or bearing false witness or committing pedophilia that we have in our hearts now. We all know these things are intrinsically wrong; we sense it within ourselves as revulsion. And yet we can rationalize and justify any behavior, overriding any and all internal laws simply because we’re free to, by deciding, for example, that no such absolute morality exists, that our interior sense of revulsion and righteous indignation against certain acts are simply the result of social conditioning, etc. We’re free to think anything, and then to act accordingly.
Adam simply exercised that same freedom. Was he culpable in some absolute, irrevocable, sense? No, which is why God never abandoned Adam but rather sought to work with humankind, leading them to perfection/salvation over time, a perfection that would ultimately involve their making the right choice.