My parents were actually grateful when I was direct with them about their memories being not so great.  I’d say this: talk with them in a patient and loving way and explain to them that this is to their benefit and tell them why.  If they get into a car accident or fall in their house, by having you assigned as their power of attorney, you can access their money to get them care.  Have a trusted 3rd party step in if necessary to help them see the reasoning behind this action.