By obligatory precaution, you should make a will for missed prayers, fasts and obligatory Hajj. (Sheikh Makarem Shirazi, issue 2330) Obligatory Hajj is paid from your whole estate. (issue 2346) The cost of making up missed prayers and fasts comes from your one-third only if you will it, or if your heirs agree. (issue 1225)

Sheikh Makarem: hiring someone to make up prayers for a dead person is not free of objection (Hajj is fine). Instead, the money is given as a gift and the person prays without payment. (issue 1213)

Your eldest son must make up the missed prayers and fasts of both his father and his mother, if they did not miss them out of disobedience and were able to make them up. If they did miss them out of disobedience, it is recommended precaution that he still makes them up. (issue 1205)