What is the expiation for the father wearing a jilbab over his ihram garments, and the mother wearing a niqab and covering her face with it, while they were in a state of ihram and entered Mecca, compelled to perform the obligatory Hajj?

Your parents are required to offer a fidyah (expiatory sacrifice) for committing one of the prohibitions of Ihram (wearing a shirt for your father, and a niqab for your mother). Each of them has the choice regarding the fidyah between fasting for three days, or feeding six needy persons, or sacrificing a sheep. It is not correct to say that they were coerced, for even if a muhrim (a pilgrim in the state of Ihram) is excused for committing a prohibition, the fidyah is not waived for him.