The first to arise in the resurrection were the prophets. The are righteous because they believe. Now Abinadi emphasizes that there will be two classes of people who will be saved with the righteous even though they did not believe. The first are those who "have died before Christ came" who were ignorant of his coming mission. This is important not only for the theological assurance of a just God who will not condemn his children for what they did not know, but is important because Abinadi and the priests are among those who are living before "Christ came." Thus Abinadi is speaking of current people who might be saved. First are the prophets (and by implication those who believe as they do) and second are those who do not believe because they have not heard. All these are saved. Next Abinadi adds "little children." Once again there is theological comfort for us in knowing this, but Abinadi's purpose in mentioning this is not to comfort future generations, but rather to condemn a current one.
In these two verses Abinadi sets us unbelievers who will be saved with the righteous - those who didn't know, or little children, who by implication are incapable of knowing (we modern readers may imply that they were younger than the age of accountability, but I suspect that Abinadi's emphasis is on their real inability to grasp the concepts rather than simply accountability).