I'll bet they'll be happy for you. Who would want their father to live on his own without love for the rest of their life? Whatever happened with their mother, they are adult enough to know that life must move on.
Just bring it up next time you see them and speak from the heart, they should be happy for you. They have their own lives now and it's only reasonable that you should start thinking of your own life and whom to include in it for your own happiness too.
Wishing you all the best.