Looking at the quoted verses:
Deu 5:9 You shall not bow down to them or serve them; for I the LORD your God am a jealous God, visiting the iniquity of the fathers on the children to the third and fourth generation of those who hate me,
This verse does not say the children are not innocent. Instead, the more probable meaning is that sometimes children are targeted as further incentive for people not to make God jealous. That and it might illustrate how hot God’s jealousy burns. Note: Calvinists don’t think God has emotions (impassibility).
Piper wants to use this verse mechanically with the next quoted verse to prove children are not innocent:
Eze 18:20 The soul who sins shall die. The son shall not suffer for the iniquity of the father, nor the father suffer for the iniquity of the son. The righteousness of the righteous shall be upon himself, and the wickedness of the wicked shall be upon himself.
Throughout Jeremiah God is said to target children. In Ezekiel, God is saying no longer will He do that but now He will distribute justice more evenly.
Ezekiel 18:20 seems like a reversal on God’s part. Note: Calvinists do not think God can change His mind (immutability).
In any case, the context of Ezekiel is to say that God will not kill innocent children, something that Piper denies is possible. Even if the children in Deu 5:9 were “guilty”, this hardly means all children are guilty and this hardly means that Eze 18:20 is saying that the children in Deu 5:9 were guilty.