What're grandparents called?!

PC • Mama to Cruz 💙 Kaia 💖 and Shaea to come 💗
We told my family this evening and neither my mom or dad want to be called grandma or grandpa. Nana is out bc we have two of those already.. and they don't want to come up with their names. (1st time grandparents)
I call my dad Vati (German for Dad) so I thought maybe this, bc it also means Papa. 
What do your kiddos call your parents? I like the name Lovie for grandma but it's Kris Jenners name. Lol 
Thoughts or ideas on names for each!?!