Kaddish for a Child Under 20

Is there a reason to say Kaddish for a child who passed away under the age of 20, since heavenly judgment is not passed on those under twenty?

This is written by the Pitchei Teshuva in Siman 376, Subparagraph 3, citing the responsa Noda B'Yehuda, second edition - Yoreh De'ah, Siman 164, which states: "In my opinion, it is also not reasonable that there would be no heavenly punishment for a person before the age of twenty, and the world would be ownerless, and he would murder his fellow when there are no witnesses, and commit adultery and all the abominations of the Lord. In my opinion, this means that in this world, the Holy One, blessed be He, does not punish him in his lifetime before the age of twenty, but after death, all of a person's deeds throughout his life, when he has reached the age of understanding, even at the age of minors, when he already knows it is a sin, he must suffer according to his deeds." Similarly, in the responsa Chatam Sofer, Part 2 (Yoreh De'ah), Siman 155, it is written: "But the truth is that from the time they become sons of mitzvah by years and signs, they are obligated in all the obligations of the Torah in the heavenly court and on earth," and it is written that only in the Aggadah do we find this distinction, and it should not be relied upon for laws. Similarly, it is written in the responsa Chavot Yair, Siman 166, and it is proven that the father says about his son when he reaches the age of mitzvot: "Blessed is He who has freed me."