That was one of the harder things I had to contemplate during my transition.
I am my children's father, nothing can change that. I will always be their parent. I personally felt that calling me mother was not appropriate. That honor is reserved for their mother and I have no intention of appropriating that for myself.
Both my kids are adults and I simply asked them to refer to me as Sandy. They introduce me as their parent and if it is appropriate for the situation they can tell them whatever they please about me.
Early on they would stumble and call me dad. It was amusing in public situations to see other peoples reaction to that. I was hardly embarrassed.
However, to my grand kids, I am first and always
Grandma Sandy. And even though one of them is old enough to understand that I was born a boy, I will always be a girl in her eyes. The younger of my granddaughters will one day ask and learn about me. I have no problem with that. But she too will have never seen me as anything but a woman.
It is really only up to you and your parent. Whatever the two of you are comfortable with, that is absolutely fantastic. The rest of society can go jump for all of me. It's their problem not yours.
And trust me, Jay, she would be very comfortable with a frank and open discussion about how she wants to be referred to and what you are comfortable with. C'mon, you know the drill!

It won't change the way you feel about her, you still love her and I think she loves you. What more needs to be said?
-Sandy