John Gill Commentary 2 Kings 14:6

"but the children of the murderers he put not to death; according to that which is written in the book of the law of Moses, as Jehovah commanded, saying, The fathers shall not be put to death for the children, nor the children be put to death for the fathers; but every man shall die for his own sin." — 2 Kings 14:6 (ASV)

But the children of the murderers he slew not Which is an instance of his clemency and goodness, and of his strict regard to justice, and to the law of God; though he might fear, these, being spared, would one time or other revenge their fathers' deaths;

according to that which is written in the book of the law of Moses, see (Deuteronomy 24:16).

wherein the Lord commanded, saying, the fathers shall not be put to death for the children, nor the children to which command Amaziah was obedient.