President John Taylor spoke of power of the atonement over children who have not yet reached the age of accountability,

“Without Adam’s Transgression those children could not have existed; through the atonement they are placed in a state of salvation without any act of their own. These would embrace, according to the opinion of statisticians, more than one-half of the human family, who can attribute their salvation only to the mediation and atonement of the Savior. Thus, as stated elsewhere, in some mysterious, incomprehensible way, Jesus assumed the responsibility which naturally would have devolved upon Adam; but which could only be accomplished through the mediation of Himself, and by taking upon Himself their sorrows, assuming their responsibilities, and bearing their transgressions or sins.”

John Taylor
Teachings of the President of the Church: John Taylor, p. 52