If Eve had not been deceived, nor given the fruit to Adam–if they had obeyed God–would one of their children or grandchildren have been deceived? If God’s plan was to prove Himself just and loving, then someone had to sin and need redemption, right?
God doesn’t control our behavior, but He knows in advance what it will be. Therefore, Eve’s disobedience was not a surprise to Him. He knew it would happen and prepared for it before the fact. 1 Peter 1:20 tells us that Jesus agreed to save mankind before the creation of the world, which means before Eve drew her first breath. But your point is well taken. Someone had to sin and need redemption for God’s purpose to be fulfilled.