The reason why the Cardinal referred to in this thread said he would not like to hear the confession of a priest if he knew in advance that the priest has sexually abused someone is this: The Seal of Confession pertains to more than divulging the contents of the penitent’s confession. It also requires that the priest or bishop who hears the confession does not act upon any of the information he learned from the penitent’s confession, nor can he speak to penitent about his sins outside the confessional.
If the Cardinal referred to in this thread did hear the confession of a priest who confessed he has sexually abused a child, you can imagine that it would put him in a position he would rather not be in, for he is in a position of responsibility over the priests in his diocese. His comment is also consistent with Canon Law which states that the bishop of a diocese cannot hear the confessions of his priests. If he did, he would not be able to act upon the knowledge he gains (in this case, suspend the priest from his ministry.)
For the same reason, the vocation directors cannot hear confessions of applicants of the seminary. This enables them to advise their bishops (in what is known as the “external forum”) about the suitability of each applicant without any conflict of interest in relation to the Seal of Confession (as this is known as the “internal forum”). Likewise, rectors of seminaries cannot hear confessions of seminarians, and pastors cannot hear the confessions of their assistant priests. If none of these measures were set in place by the Church, it would prevent bishops who have a duty of care for priests, or those being formed to become priests, to exercise the appropriate authority they have over them that is required.