You might consider opening your conversation with one of the following questions.
"I know you can manage on your own, but I'm here if you need me. Have you thought about where you'd like to live if you needed more help?"
"Have you thought about what you would do in an emergency?"
"Could you give me the name and phone numbers of your (doctor, lawyer, accountant, or others that might help manage their affairs) in case there is an emergency?"
"Dad, since Mom pays the bills and you prepare the taxes, do you know each other's filing systems? I'd like to know, too, since Mom might need help if anything happens to you."
"Mom, I have recently been looking over my life insurance policies and will and have made sure that they are filed in a safe place. Have you and Dad done this?"
"I want to talk with you about the future while you are both healthy. You always took good care of me. Someday it might be my turn to take care of you, and I want to know what's important to you."