I had a gramma I adored. She was tiny (I was taller by middle school) and had a Boston accent. I loved to visit and help her make jam or listen to her play the piano. We would ice skate across the street or, in the summer, pick blackberries. She had a special silly nickname for me that I sometimes still hear her say in my head. But in all the boxes of old photos I have, I don't have any of just her and me. I wish I did.
My mother-in-law died while my children were still very young. She was the one we counted on to babysit and the kids were always so happy to see her come to the door. Her patience with them was endless. But, sadly, they don't really remember her (or either of their grandpas).
Grandparents dote on their grandchildren, it's such a special bond. I love it when grandparents are included in a session, they are usually so excited to be a part of this memory-making day (and they have that magic touch that sends away pouty faces). Why not have a special session just for them? I can't think of a better way to show your appreciation for their love and support. Casual or fancy, it will become a treasure for all involved.