Is what we did—exiting the state of ihram and abandoning the 'Umrah due to the daughter's fatigue and the father's advanced age—considered a grave error necessitating the sacrifice of an expiation?

There is no sin or fidyah (expiatory offering) upon one who commits prohibitions of ihram out of ignorance, because their ihram remains valid. They must continue their rituals and abandon the prohibitions once they become aware of them. If they persist in committing them after gaining knowledge, then they are sinful and the fidyah becomes obligatory upon them.