My husband and I both have POA for both parents, trust in place? My father has wishes for us to be guardians if he passes. Any thoughts?

Can a letter be written of my fathers wishes for his wife who has dementia who has not been diagnosed yet on paper that she is not competent to take care of herself, finances, meals etc. Is it a legal document if he tells us the care he wants for her so other family members don't jump in and move in someone, argues with us they don't want her at home, or they don't want her admitted to a facility? Can he state he wants us to be the guardian for her/or him if something happens to him or her say tomorrow ? He doesn't want to have to go to court and declare her incompetent and fight over who upholds their wishes for each other but he does want to state no family moves in the home, no family relative or friend will take care of her, he wants us to keep her in the home and hire an outside experienced nurse/caregiver of our choice and desires no family dispute over their care. Being we have POA I know we have some say but we want it in writing and notarized so there are no questions , verbal details can be forgotten or incorrectly taken if not in writing. Can we sit down and write this up for both parents, we would like to ask how to word it or is this something an attorney has to draw up also and then he calls both children in and discusses it and gives each a copy. Thoughts would be appreciated.