The general sense of this verse is simple. Anti-Nephi-Lehi is grateful for the gospel that they have been given. The closer reading is more difficult, and doesn’t have a clear explanation. He notes that: “he has made these things known unto us beforehand…” Before what? The only clue we get is that we are told that God loves the parents as much as the children. What difference is there between parents and children, and why should we assume anything other than that God would love both?
While it is not completely clear, it appears that a reasonable explanation for this verse is that “these things” that were made known were not the tenets of the gospel, but rather the awareness of the spiritual danger to the parents of taking up arms again. The children had no such stain nor danger, Indeed, these children will become the stripling warriors that Helaman leads into battle. The “beforehand” refers to the knowledge of this danger prior to the situation that might require force of arms, the coming attack.