Verse 12 established that little children are alive in Christ, and Mormon repeats that concept for emphasis. The argument here is one that he has already propounded, but here it is in different words. The slight expansion here is the comparison of the little children to those who are without law. The ability to understand and properly judge according to the law of the gospel that is given to us is our blessing in that it allows us to progress.
It is also our curse in that it allows us to sin. Without that law, however, there is neither possibility. We cannot become perfect (Matthew 5:48, 3 Nephi 12:48) without the law, but neither can we be condemned. Those two conditions are part and parcel of the effect of agency, an effect that cannot be fully active until we are able to know the difference between good and evil. Symbolically, at the age of accountability, we too eat the fruit of the Tree of Knowledge of Good and Evil.