with my DF's mum, I just put Dear Mum/Celia/Nana. As we all call her different things...I call her by her name, DF of course calls her mum, and she wants DD to call her Nana.
With my parent's card, because my brothers are on there as well, I just put dear mum, dad, & brothers names. Easier that way otherwise the card would be taken up by what we each call them!