I think those names are a little ridiculous, but I don't really like the idea of dictating to grandparents what their titles are.
My XMIL names herself something that sounds very similar to Nanny-moo, she used to call herself that to the kids all the time, signs off cards etc with it. I always just refer to her as Nanny. After a while, and especially as the kids got older, they dropped the silly name and just call her Nanny.
So that's what I recommend OP.