§1. Minors below the fourteenth year of age and those of limited mental capacity are not allowed to give testimony; they can, however, be heard by a decree of the judge which declares such a hearing expedient.
§2. The following are considered incapable: 1/ the parties in the case or those who stand for the parties at the trial, the judge and the judge’s assistants, the advocate, and others who assist or have assisted the parties in the same case; 2/ priests regarding all matters which they have come to know from sacramental confession even if the penitent seeks their disclosure; moreover, matters heard by anyone and in any way on the occasion of confession cannot be accepted even as an indication of the truth. Art. 2.
Source: Code of Canon Law (Vatican.va)