SB 7.5.9
vatsa prahrada bhadram te
satyam kathaya ma mrsa
balan ati kutas tubhyam
esa buddhi-viparyayah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Dear son Prahlada, all peace and good fortune unto you. Kindly do not speak lies; just reply with the truth. These boys you see are not like you, for they do not speak in a deviant way. How have you learned these instructions? How has your intelligence been spoiled in this way?
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Prahlada Maharaja was still a boy, and therefore his teachers thought that if they pacified the little boy he would immediately speak the truth, revealing the secret of how the Vaisnavas came there to teach him lessons in devotional service. It was surprising, of course, that in the same school the other boys of the Daityas were not polluted; only Prahlada Maharaja was supposedly polluted by the instructions of the Vaisnavas. The main duty of the teachers was to inquire who those Vaisnavas were that came to teach Prahlada and spoil his intelligence.