I have three grown children, too. I have already told them that if anything happens to them and they are in a coma or bedridden it will be ME who takes care of them. Their spouses can help, if they want to, but I have offered to do the work myself. My kids know that means I love them.
Have the children and their spouses agreed to assign guardianship over to you?
I feel the same way you do. If anything happens to one of my own I will be there to take care of them. I did it for my mother and I would do it for my children as well because that's what love is. Giving freely of ones self.
And you know what love means . . .
I took care of my Mom and it was a privlege I will always treasure.