The way I see it, you're either going to have to create harmony between the Yorkie and the grandchild, or separate them until the child is at least old enough to understand the dangers that can befall a little dog and that all dogs bite.
If I were to choose to separate, I would still want the dog and baby to have 'near-ness' to each other so they can get used to scents, sounds, etc., associated with each other. A baby crying in pain due to a gas bubble or constipation could set your Yorkie quite on edge, since likely no one will be paying attention to the dog during those times because they're concentrating on baby's needs instead.
If I were to choose to create harmony...which sounds like me...lol, I would proceed cautiously. If my Yorkie had never been around babies, I would use whatever "prep time" I have available to take my Yorkie places it could hear and see babies of all ages
in the presence of others who rather ignore their sounds as 'normal' so the dog gets the idea...picks up the 'vibes'...that the noise is no big deal. Otherwise, a wailing infant could be stressful to the Yorkie. I'd definitely allow the baby and the Yorkie to 'meet' on the first visit, although I'd not let them get close enough for the dog to do more than 'niff at the baby. I'd keep them close but separated so they could get used to each other's presence, and hold the Yorkie as much and as often as the child.
Depending on how often they were together, I believe it's possible to get a 'feel' for how they are going to 'take' to each other. The big thing for me would be to make certain the dog and baby weren't scared of each other...indifference would be a good start until baby is old enough to learn about dogs enough to hopefully not trigger some primal instinct of the dog. Little children make sudden movements, sudden and loud vocalizations, and some dogs are off-put by these things. Dogs bark suddenly and may run toward the child, which may frighten the child and start an uncomfortable situation, too.
I had my Yorkie-Poo 9-10 years before I had my first baby, and had no difficulty getting him used to the baby. He 'adopted' the child and protected his 'ownership' of him in the same way that he 'protected' his 'ownership' of me. lol But, I also made certain baby did nothing harmful to Stormy either, and by the time my older child was a year old, the two were great buddies.
Good luck...and congratulations!