D&C 68

The Lord said -

"...inasmuch as parents have children in Zion, or in any of her stakes which are organized, that teach them not to understand the doctrine of repentance, faith in Christ the Son of the living God, and of baptism and the gift of the Holy Ghost by the laying on of the hands, when eight years old, the sin be upon the heads of the parents."  (Doctrine and Covenants 68:25)

I specific note that the word "sin" is singular suggesting to me the sin of the obligation to teach our children truth.  We must remember that our 2nd Article of Faith is that man will be punished for their own sins.