1970s or earlier animated – girl sees herself as a monster in a mirror and turns into the monster

Hi – I only remember one scene from this movie or animated show.  I saw it on television one afternoon I believe in the early to mid 1970’s.  I know the show was animated and believe it might have been stop motion animation, but I’m not positive about that.  The show was in color and in English.

In the scene I remember a girl in being held in a castle or creepy house.  A witch  has the girl look into a mirror, and the girl sees a version of herself as a monster.  Then the girl realizes she’s turned into the monster version of herself (about which she is very upset).

My recollection is that there is another child in the house (I think a little boy) about the same age as the girl. He gets her to realize she’s not been a very nice person and helps her to transform back to her real self.

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