Lehi blesses the lineage of Laman, addressing them as though they are his own adopted children (they are at least “adopted” into a blessing that should have been pronounced upon his son, Laman). He begins with the most hopeful statement he can make: that if those children are brought up “in the way [meaning the gospel] ye should go” they will “not depart from it.” Lehi is echoing a phrase that they would have known from Proverbs 22:6: “Train up a child in the way he should go: and when he is old, he will not depart from it.”
Lehi knows that his grandchildren are not inherently bad and foresees that grave eternal consequences will fall upon those who do not teach these children “the way.” This teaching is repeated in Doctrine and Covenants 68:25: “Inasmuch as parents have children in Zion… that teach them not to understand the doctrine of repentance, faith in Christ the Son of the living God, and of baptism and the gift of the Holy Ghost by the laying on of the hands, when eight years old, the sin be upon the heads of the parents.”
In essence, Lehi’s blessing is a negative one—not that good things will befall them, but that the cursing due to their disobedience will be deflected to their father (who, as patriarch, is responsible for his lineage). Lehi mitigates, though he cannot shield them from, their fate. The blessing that the lineage will not perish is fulfilled temporally in Book of Mormon history and beyond, but Lehi’s earlier prophecies that the gospel would be restored to his descendants indicates that he saw a spiritual dimension to this blessing as well.
Variant: Joseph Smith struck the word right from the phrase “I know that if ye are brought up in the right way ye should go ye will not depart from it.” Skousen notes that the phrase “right way” occurs frequently in both the Book of Mormon as well as the King James Bible. I suspect that the elimination of right was an attempt to more closely align the text to what appears to have been the model, Psalm 22:6: “Train up a child in the way he should go: and when he is old, he will not depart from it.” The text from the original manuscript which has the “if ye are brought up in the right way” may better reflect the Book of Mormon vocabulary rather than simply the King James text.