I have an adult disabled child. I'd like to be able to choose the person in charge of his or her interests in the event of my death. Is this possible?

Yes. It is possible to draw up a "future protection" mandate before a notary, in which you appoint a trustworthy person to look after your child's interests. You must be materially and emotionally responsible for your child at the time the mandate is signed, and the child must be unable to look after his or her own interests due to an alteration in his or her mental or physical faculties preventing him or her from expressing his or her wishes (article 477 C. civ.).