I am Granny to all my tiddlers, although my 'official' title is actually Little Granny, as there is still a Big Granny (great-grandmother). This has followed on from when my sons were small, and her mother was still alive (great-great-grandmother). My parents were always Grandma and Grandpa to my boys, which they loved, and I'm sure, had they still been with us, the tiddlers would call them the same.
The DiL's mums both opted for Nanny, so it's always been clear to the tiddlers who everyone is. The Grandads are just that...Grandad (or Grandan as DGS3 calls them).
My eldest 3 GC don't have an immediate Grandad sadly, as my DiL's Father has passed away, and my ex has no contact at all (his choice), but they have their Uncles and cousins, and Big Grandad (spouse of Big Granny obviously) so have not missed out on other male members of the family.
The only odd thing about the whole set-up is that Big Granny and Grandad are actually my ex's parents, but they are very much part of our lives, and we all see them at least once a fortnight, adults and tiddlers alike.....he sees his parents maybe 2-3 times a year, in spite of living in the same town as them.