Then why did God punish them for it? If my children didn't know the difference between good and evil, I sure wouldn't punish them. I'd teach them.
I am sure God wanted to teach Adam and Eve the difference between good and evil in his own time. Satan pushed the time forward. When he misled A & E he did (almost) irrepairable damage, he caused them to sin. That is why they had to leave the garden, God couldn't be with sin. The world outside the garden was not insulated, God didn't punish A & E, He reiterated the consequences of not being in true fellowship with Him. The earth is not easy to tend, you will now feel pain, equality between the sexes is gone, you will know evil. So therefore because I love you, I will number your days.
Our time spent on this earth gives us ample opportunity to learn the difference between good and evil. We are also given the opportunity to choose good over evil. We are still given that choice away from true (complete) fellowship with God. Why? Because God is just, not wanting any soul to die.