See the notes for their daughter Mary regarding mention of her having a sister Susanna. This is clearly the daughter Mary attributed to Joseph Packard and Mary Willis. So, either there was a Susanna in this family, or ALL of the children attributed to this family by the Mitchell and Packard sources probably belong to the family of Joseph Packard and Susanna Mitchell.
P.S. The above was written before the wills of the two fathers were found, both naming a daughter Susanna. Thus the four baptisms found in West Bridgewater, including this Susanna, exactly match the children named in the will of Joseph Packard, husband of Mary Willis.