In Latter-day Saint doctrine children are to be instructed in the principles of the gospel and baptized when eight years of age (D&C 68:25-27). They are then responsible to adhere to the teachings of the Church relative to obtaining salvation. Before that time they are considered "infants" or "little children" and are not required to be baptized. They are considered "alive in Christ" and are "whole" (Moro. 8:8-12; JST, Matt. 18:10-11).
Although children, with all the rest of mankind, feel the mortal "effects" of Adam's transgression, they (and all others) do not have any mystical stain of original sin upon them. Adults must have their own personal sins remitted by repentance and baptism (John 3:5; Acts 2:38; Moses 6:57-62), but "the Son of God hath atoned for original guilt, wherein the sins of the parents [both Adam's and their mortal parents'] cannot be answered upon the heads of the children, for they are whole from the foundation of the world" (Moses 6:54).
The prophet Mormon taught: "Listen to the words of Christ; …the curse of Adam is taken from them in me, that it hath no power over them…. It is solemn mockery before God, that ye should baptize little children" (Moro. 8:8-9). The Lord instructed Joseph Smith that "little children are redeemed from the foundation of the world through mine Only Begotten; wherefore, they cannot sin, for power is not given unto Satan to tempt little children, until they begin to become accountable before me" (D&C 29:46-47).
This unconditional benefit of Christ's Atonement saves all little children regardless of race, color, or nationality, for "all children are alike unto me" (Moro. 8:17). They all begin their mortal lives pure and innocent (D&C 93:38), and "little children also have eternal life" (Mosiah 15:25).
If they die while in this state of innocence and purity, they return to that God who gave them life, saved, and fit for his company. They are in a "blessed" condition, for God's "judgment is just; and the infant perisheth not that dieth in his infancy" (Mosiah 3:16, 18). The Prophet Joseph Smith saw in vision "that all children who die before they arrive at the years of accountability are saved in the Celestial Kingdom of heaven" (D&C 137:10; TPJS, p. 200).
All that is said of infants and little children applies also to those who may be adults in physical body but are not accountable mentally (D&C 29:49-50).
Concepts outlined in scripture and by the prophets clearly demonstrate the marvelous uniting of the laws of justice and mercy because of the Atonement: none are eternally disadvantaged by noncompliance to gospel laws or ordinances they do not know or are not capable of understanding and thus cannot comply.