Real children never appeared on the show, as this would have shattered the carefully crafted illusion that he was a child (the show's popularity overseas arose, in part, because audiences unfamiliar with British showbusiness believed he really was a child). So he discussed his pal Ozzie, and his schoolboy friends in the Black Hand Gang (who would punish any member caught in the company of a girl), but the characters he spoke about were never actually heard themselves.
The show's authenticity was maintained by avoiding real children, as it would have contradicted the illusion of him being a child (the show's international success stemmed in part from audiences believing he was genuinely a child).  So he discussed his friend Ozzie and his schoolmates in the Black Hand Gang (who enforced strict rules against fraternizing with girls), but the voices of these characters were never actually heard.