One of the complexities I see, whether a person transitions before or after becoming a grandparent, is that their grandkids may look in the photo album and say, "Who's that?" and you say, "That's Pops/Nan," or whatever term you choose and, if they are old enough, might say, "But that's a man/woman." You then have to say, "Well, see, Pops/Nan was born male/female but transitioned."
People say, "Oh, but nothing's changed," and I think it's more nuanced than simply, "Yes, it has, or no, it hasn't." A trans person still has the same opinions, may still have the same hobbies and interests as they had prior to transitioning, but how they are seen in the world may be different. I think, though, kids can be understanding enough to know that Pops/Nan was a Mum/Dad who transitioned and still love them regardless, or at least hope they can. I am hoping that with my niece and nephew that they'll be able to say, "My relative used to be my Uncle."