I remember seeing an opening to this show that had children getting ready to perform, and they tried on (or put on) the drama and comedy masks, and it scared the living daylights out of me. So of course, I'd like to see it again. Does anyone have any idea of what I'm talking about?
You are close, but the credits you remember don't belong to the Festival, but to the "CBS Children's Hour", a short-lived anthology of children's dramas. The short films in the show originally aired between 1969-1970 during the period when the Festival was on hiatus, so it's understandable that you would think it was part of the CFF.
The most famous of three films was "J.T.", which was so acclaimed it aired on primetime the week after its origional broadcast and at various times years afterward. The other two short films were "Toby" and "Summer is Forever". From what I understand, it wasn't cost effective to produce these dramas, so CBS went back to their old format of using educational films and licensing films from other countries for the CFF.
You can watch the shows as they originally aired, with the opening you remember, at the Paley Center for Media (located in New York City and Los Angeles). It may be possible to obtain the episodes from the "CBS Children's Hour" (J.T. seems readily available), but I'm not sure they will include the original opening. The shows were produced by CBS, so I suppose they could release or even broadcast them at some point if there was sufficient interest.