2. Since little children and those who because of mental or physical deficiencies never mature in the moral or spiritual sense are not accountable (see D&C 20:71; D&C 29:49-50; D&C 68:25-27), it is impossible for them to sin. Without accountability there is no sin. Without sin there is no need for repentance and baptism.
“Little Children Are Whole for They Are Not Capable of Committing Sin”
It should not be thought that this verse teaches that little children of the mentally deficient are incapable of doing anything wrong, mean, or dishonest. They are “whole” and incapable of committing sin not because of any inherent perfection or goodness on their part but by reason of the Atonement. They are incapable of sinning because they are not accountable for their actions. This means that whatever actions they may do that are mean or dishonest are not counted against them as sins.