Mosiah 3:19 is so often quoted by itself that we miss the important context for which it forms a conclusion. Two previous threads combine in this conclusion. The first is the state of humanity after the Fall. The second is the contrast to the fallen state of humanity and the tabula rasa of the infant. Benjamin has explained that little children, as those who do not have sufficient culpability to intentionally obey or disobey, are not guilty. Yet, they would be condemned under the law.
For both examples, atonement occurs through the Messiah’s sacrifice. While the law itself might condemn an innocent child for the mere fate of being a human in a fallen state, the atoning act of the Messiah, the blood of the Lamb, will reconcile the law with the blessings of mercy. Similarly, the sacrificial blood of the Messiah will provide the way whereby humankind, who are culpable through their agency, will have the ability to have their sins removed and return to the covenant.
The image of the child is that of the child who is learning, but not yet responsible for their sins. Through the remission of sin, humankind may return to that more innocent state and, in effect, start over in learning to obey our Heavenly Father.