Second Nephi 2:25 should never be quoted without also quoting verse 26. They are inextricably bound together as two pillars in the great plan of salvation. Yes, Adam fell that men might be, but “the Messiah cometh in the fulness of time, that he may redeem the children of men from the fall.” It was all planned from the beginning: the Fall and the Redemption.
The children learn through righteous use of agency, through their proper choices to act for themselves and make progress toward becoming eternal fathers and mothers.
The phrase “fulness of time” refers to the fulfillment or completion of all righteous principles and practices known up to that time. The final dispensation is known as the “fulness of times” for the same reason (D&C 27:13; 121:31; 124:41). The last dispensation becomes the fulness of all other times. The period of the Messiah’s mortal ministry is known as the “meridian of time,” or the high point of earthly time.