Since it was the desire of the Lord that Pariksit Maharaja be put into that awkward position so that he might be delivered from material existence, then why was a brahmana's son made responsible for this offensive act?
SB Canto 1
Then the ṛṣi prayed to the all-pervading Personality of Godhead to pardon his immature boy, who had no intelligence and who committed the great sin of cursing a person who was completely free from all sins, who was subordinate and who deserved to be protected.
Everyone is responsible for his own action, either pious or sinful. Ṛṣi Śamīka could foresee that his son had committed a great sin by cursing Mahārāja Parīkṣit, who deserved to be protected by the brāhmaṇas, for he was a pious ruler and completely free from all sins because of his being a first-class devotee of the Lord. When an offense is done unto the devotee of the Lord, it is very difficult to overcome the reaction. The brāhmaṇas, being at the head of the social orders, are meant to give protection to their subordinates and not to curse them. There are occasions when a brāhmaṇa may furiously curse a subordinate kṣatriya or vaiśya, etc., but in the case of Mahārāja Parīkṣit there were no grounds, as already explained. The foolish boy had done it out of sheer vanity in being a brāhmaṇa's son, and thus he became liable to be punished by the law of God. The Lord never forgives a person who condemns His pure devotee. Therefore, by cursing a king the foolish Śṛṅgi had committed not only a sin but also the greatest offense. Therefore the ṛṣi could foresee that only the Supreme Personality of Godhead could save his boy from his sinful act. He therefore directly prayed for pardon from the Supreme Lord, who alone can undo a thing which is impossible to change. The appeal was made in the name of a foolish boy who had developed no intelligence at all.
A question may be raised herein that since it was the desire of the Lord that Parīkṣit Mahārāja be put into that awkward position so that he might be delivered from material existence, then why was a brāhmaṇa's son made responsible for this offensive act? The answer is that the offensive act was performed by a child only so that he could be excused very easily, and thus the prayer of the father was accepted. But if the question is raised why the brāhmaṇa community as a whole was made responsible for allowing Kali into the world affairs, the answer is given in the Varāha Purāṇa that the demons who acted inimically toward the Personality of Godhead but were not killed by the Lord were allowed to take birth in the families of brāhmaṇas to take advantage of the age of Kali. The all-merciful Lord gave them a chance to have their births in the families of pious brāhmaṇas so that they could progress toward salvation. But the demons, instead of utilizing the good opportunity, misused the brahminical culture due to being puffed up by vanity in becoming brāhmaṇas. The typical example is the son of Śamīka Ṛṣi, and all the foolish sons of brāhmaṇas are warned hereby not to become as foolish as Śṛṅgi and be always on guard against the demoniac qualities which they had in their previous births. The foolish boy was, of course, excused by the Lord, but others, who may not have a father like Śamīka Ṛṣi, will be put into great difficulty if they misuse the advantages obtained by birth in a brāhmaṇa family.